There are different types of fail pictures, but on the whole, fail pictures do not deliver any extra humour to a photo. Often, it detracts from the humour, or it simply states the obvious.
Example 1: Not adding anything aka LULZ FAIL
The purpose of the written fails seem to point out that the two cars are smashed by trees, yet there is one car that is perfectly fine. The image clearly indicates that the size of the two hummers carry the entire burden of the tree, and leave the one car unharmed. One does not need "FAIL WIN FAIL" slathered onto the image. But, I can find this acceptable, but it is
overused.
Example 2: Detract from the lulz
This is a well known image that points out the fallacy of the "mission accomplished" banner. Again, one can quickly see the "mission accomplished" banner. So this image appears to be the genius of some internet poster, who had the brilliant idea "Wow! I bet no one thought of putting FAIL on this before! Surely it makes it more funny!"
A similar event happens with demotivators of already well known internet memes and pictures. Classic images are converted into demotivator form. As in example 1, it adds nothing new, but it further extends the problem by being completely unoriginal or innovative. People familiar with the internet and funny picture threads have seen countless numbers of images- there is something to be said for seeing something new.
For reference, here is already iconic image of a "failing" security camera, that is now in demotivator form.
Example 3: Slapstick lulz
Somehow, the addition of "FAIL" doesn't add to the image. One can find humour in seeing the odd positioning of the ball and face, without needing additional textual clarification.
Example 4: There is no lulz
I'm sure the creator of this fine image felt proud for his unique use of the words "Epic Fail" to describe General Motors. He probably looks forward to the day where he, as an old man, can explain to children how he once created an incredibly intelligent image. What a joyous day it was for him! However, the only joke appears to be a connection between GM and failing. To me, this isn't original.
So, to quote encyclopedia dramatica,
"Ever since the the creation of the Universe, to the times of Jesus Christ, and even in modern times, there have always been fails. For example, you.
Many people whom have recently discovered the intarwebz find this meme, EXTREMELY lolworthy and so they enjoy sharing it with their friends by screeching "FAIL, LOLOLOLOL OMG DIZ IS FUNNY ROFL", whenever they mess up, even slightly with any of life's given problems. These people, of course are retards and should be avoided at all costs."