Photoshops are discouraged, but derivative works and parodies (provided they are of sufficient quality and don't run into trouble with other guidelines) are fine. In spite of the title of the pool, this straddles the line between a photoshop and a full parody, so there would be some leniency, and these are generally done in the same style as the original, but I think some of the others really are close to basically being a nude filter and maybe deserve some double-checking...
One of the major reasons photoshopping is problematic because it causes a clash in the art styles, and a lot of photoshopping is done by people not as skilled as the original artist (which is why they are photoshopping someone else's work instead of making their own). (Also, you want to credit the right artist for a work...)
