As you probably know, most of the pictures you're seeing here have been created using Adobe Photoshop 4/6. This program is the standard for things like this and for good reason; composites like this can be created in minutes. Practically all the "base" photos are from the Usenet newsgroups.

Note that I say "most" were created with Photoshop. Not quite all, but I'd challenge you to accurately pick out which ones are "Photoshopped" and which are not. Some are obvious, but not all.