The enlightening image below is of a lightning strike slowed down at 10,000 frames per second. It can be seen that the most intense flash produced from the lightening occurs in the direction from the ground up. Why does this final "ground-up" strike occur and why is it so much brighter and faster than the initial part of strike heading towards the ground?
Does lightning strike from the sky down, or the ground up?
The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of spurts. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in about one-millionth of a second - so the human eye doesn't see the actual formation of the stroke.
The reason is that when cloud-to-ground strike approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field and the "downward leader" strike creates bridge for the "return stroke"; this per the wiki page for Lightning.
Cloud to cloud and Intra-Cloud Lightning
Might be worth also noting that cloud-to-ground is not as common as Cloud to cloud (CC) and Intra-Cloud (IC):
Lightning discharges may occur between areas of cloud without contacting the ground. When it occurs between two separate clouds it is known as inter-cloud lightning, and when it occurs between areas of differing electric potential within a single cloud it is known as intra-cloud lightning. Intra-cloud lightning is the most frequently occurring type.
Ground-to-Cloud
Appears that ground-to-cloud is possible, though normally only a result of a man-made object creating "unnatural" electric potential, and is the least common type of lightning.
Lightning happens when the potential difference between the clouds and the grounds becomes too large. Once the voltage reaches a critical strength, the atmosphere can no longer act as an electrical insulator. First, a stepped leader is created at the base of the cloud which is a channel through which electrons in the cloud can travel to the ground. But while moving towards the ground, it searches for the most efficient(minimum electrical resistance) route possible. It does so by traveling 50-100 meters at a time then stopping for about 50 microseconds, then traveling another 50-100 meters. In this process it also branches out looking for the best route. As the stepped leader gets close to the ground, a positively charged traveling spark is initiated on some tall object (trees, towers etc) on the ground. The traveling spark moves upward and eventually connects with the stepped leader. Once the stepped leader and the traveling spark have connected, then electrons from the cloud can flow to the ground, and positive charges can flow from the ground to the cloud. This is known as return stroke. But this flow unlike the flow from up has a well defined shortest route now. This massive flow of electrical current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly superheats the completed leader channel, forming a highly electrically-conductive plasma channel. The core temperature of the plasma during the return stroke may exceed 50,000 K, which makes it shine so bright.
In your image above as well, you can see a lot of time is lost in looking for the route, whereas the return stroke just runs through the well defined channel and hence is much faster.
In the video, the downward strike created an ionized path. The brightness was less as the current was less because it was travelling through un-ionized air. The upward strike took the ionized path. The upward strike's current (brightness) was much greater as the electrons flowed along the ionized path as it had the least resistance. The current equals the voltage divided by the resistance. The less the resistance, the higher the current.
The apparent speed difference is because the upward strike took a more direct path. The camera shows a two dimensional image and does not show the motion of the lightning away from or towards the camera. In a video when you see an object coming directly towards or away from the camera, you see the object get larger or smaller. The size differences in the video are not large enough to show the direction towards or away from the camera.
I'm pretty sure that lightning can go either way. We did the calculation in physics class, a bit of a back of the envelope calculation to be sure, but we came out with an electric field of 300,000 V/m being the electric field needed for lightning to form.
What that means is, an electric field is created between the clouds and the ground by a build up of electrons on one of the surfaces. I believe that the electron build up can occur on either side. Once the field strength gets high enough, electrons begin to "leak" from one side of the air-gap (air does not normally conduct electricity unless it is highly charged) and that is what lightning is. You see the same affect in a capacitor that is overcharged, only instead of a thin sheet of paper, you are inside the insulator between plates. That is why you get a lot of noise (thunder of course), see a lot of light, and wireless communications and some electronics are affected.
I won't repeat something provided in other answers. The only thing i would like to add is that we don't see a Ground To Cloud Lightning.
Theoretically speaking Ground To Cloud could be possible since lightning is a discharge between 2 points that have extreme voltage difference. So it is not a mystery "why this happens" as said in this video, but 'how this happens".
Is it a case of Geology of the location (like Lake Maracaibo or the lake of the thousand lightnings)? Yes and it is needed to examine more thoroughly since some effects do happen more frequently in certain geographical locations all it is needed to be understood is WHY (and it could be not just one factor).
Map of average yearly lightning strikes per sq km from 1995 to 2013 (Credit: NASA)
But to return to this picture
and please take a good look. Notice the bright flash on the top left corner at the beginning that spreads into several leaders reaching for the ground. No ground path been established until two of those leaders reach the ground. From those two paths one of them has less resistance leading to a momentary current path among the cloud and the ground. I do think that in this case we see a Cloud To Ground lightning but of these two types, positive and/or negative lightning on an already established path. The image is taken from an High Speed or Ultra High Speed Camera, capable of 10k-1m frames per second, what we see is very slow motion of a cloud discharge.
Furthermore one thing you should have in mind is that lightning is not just a static electricity discharge.
This discharge may produce a wide range of electromagnetic radiation, from very hot plasma created by the rapid movement of electrons to brilliant flashes of visible light in the form of black-body radiation. Lightning causes thunder, a sound from the shock wave which develops as gases in the vicinity of the discharge experience a sudden increase in pressure. Lightning occurs commonly during thunderstorms and other types of energetic weather systems, but volcanic lightning can also occur during volcanic eruptions. Wikipedia
Above is written that the discharge itself may produce wide range of electromagnetic radiation.
The majority of that radiation should be produced once the Cloud To Ground path has been established. In case we do have a combination of positive and negative lightning on that path then perhaps a differently filtered view of this lightning (microwave, infrared, ultraviolet, or even X-ray range) would have been more enlightening.
I actually don't think that all the radiation comes from the discharge itself as written above. It should be a combination of static electricity and radiation even before the discharge, only to be amplified at the moment of discharge.
An even more peculiar case than Ground To Cloud lightning is also the case of Ball Lightning. This video shows something that could be identified as ball lightning. One of the theories on how ball lightning occur indicate that Si element on soil could be a factor.
EDIT
In order to prove my point on this optical illusion in the above image i add a link of another optical illusion, of course on that video the train has one and only course but we see it to have both, in cases like that the human brain finds other ways to determine the direction.
It doesn't.
Based on observing this and other slow motion videos of cloud-earth lightning I observe the following:
Some low-intensity "pilot lightning" starts in the cloud, usually only at one point or at most a few points, and propagates in many directions away from that point, branching out much like a river delta or a tree branch.
The general direction of propagation is towards the ground, but not very strongly. The most consistent trait of the movement is away from the origin, but not even that is absolute (there are a few loops and upward branches).
Once one of the branches touches or gets close to the ground an intense arc forms between that point on the ground and the origin of the "pilot lighning". There is no clear indication of a direction, partly because of the unavoidable camera overload.
Hypothesis
The pilot lightning starts where the gradient of the inhomogeneous electric field is steepest, somewhere close to the concentrated collection of charge in the cloud. It does not start on the ground because the ground conducts comparatively well so that the charges dissipate, lowering the electric field strength.
The pilot lightning's propagation is driven by two factors: The electric field and a feedback mechanism. The feedback happens because the plasma corridor created by the emerging lightning bolt conducts electricity very well and thus facilitates propagation once it has started: Electrons and ions are accelerated and extend the corridor at its front. Apparently there is a chaotic element to this, possibly because turning air into plasma is an explosive process. This multi-directional explosive expansion is also responsible for the branching.
That the lightning roughly follows the field gradient explains the general direction. The feedback sustains the lightning once it has started. The explosive expansion of the heated air explains its chaotic parts.
When one of the pilot lightning branches gets close enough to the ground that an arc to the ground is completed there is a completely conductive connection between the separated charges which will lead to the runaway discharge known as a lightning bolt. (It's a runaway process because higher currents increase the arc's conductivity by turning more air into plasma, so the current grows until the charges are exhausted.)
I do not think that the eventual main lightning bolt has a different "direction" than the pilot lightning, and I cannot find observational evidence. Even at 100,000 frames per second the intensifying appears instantaneous, and the main bolt is so bright that cameras are overloaded.
It may appear to the human perception as a "back strike" because it starts once the pilot lightning touches ground, much like a bounce-back event. But I think it's just that suddenly a lot of charges flow where only few charges were flowing a millisecond earlier, in the same direction.
