I think fireattack has it in a nutshell and I agree. Included with this, however, I would say that there is implicit in this that posters include, at a minimum, the artist's name and source of the picture so that those who want to see the variations can search for them on other sites. As far as what Checkmate has said I think those could also be regarded as child posts if the original post was posted a long time before and deleting the original would cause problems otherwise I'd say just delete the inferior copy in favor of the better one.