The conclusory stance of the previous post: utilizing the image of somebody else in a slanderous way for monetary gain without their consent is highly inappropriate and deserving of monetary compensation. I find this especially true if the image was only slightly modified by the user: for instance, Alison's photo was minimally modified with a sexually-indicative and humiliating caption.
However, in the case of "anime music videos" and other meticulously-assembled videos combining pre-made music with snipits of pre-made networking, I find these to be perfectly legal and void of harm to the orignal video/music artists. These free-lance internet collage artists take original works and painstakingly cut and rebuild them into something truly their own without monetary gain (although internet popularity has some degree of unnamed value). These collaged music videos don't detract from the sales of the original artists. If anything, they boost their popularity by advertising the works all across the internet to millions of viewers! I myself have come across songs in personally-made music videos on YouTube that I didn't know before, found myself liking, and then proceeded to purchase them on iTunes.
If a person can take the images of other popular works of visual art, cut them up, reassemble them, and call the finished piece their own, why can't the same be done with videos and music recombined in a completely unique way?