When it comes to parody it's perfectly legal to take a song and put your own lyrics to it (if you make the music yourself or buy a royalty-free karaoke version of it). And you can make fun of commercials (such as SNL's use of brand names in their ads).
So, is it legal to take a photo of, say, Muhammad Ali standing over Frazier and paste your own (or a friend's) photo on it? We've all seen it, and it can be pretty funny (and realistic). I've even done it myself (but not published it). I took a photo of a friend from work and put it on the famous Dorothea Dix photo of the mother and her children during the Great Depression.
So, is it legal? It's certainly fun, but if it's legal I might want to get more into it. People think it's funny, especially if it looks realistic.
Another example would be putting your buddy's picture onto Lee Harvey Oswald in the famous shot-in-the-gut photo.
Or the pinnacle of the genre: Lootie








