As I was thinking a bit too deeply about these, however, I realized they are only funny if you know what they mean; in this case, who the caricatures represent. No names are given, so you just have to know who they are.
The rest of this post is a 'spoiler,' so if you haven't looked at the pix yet, you might want to.
I find it interesting that the physical appearance of certain famous people have become such a part of our "cultural literacy" that it's not even necessary to identify them by name in a photo or illustration, even in an absurd caricature like these. Several of these caricatures are generic fantasy-figure types, obviously, but those who represent real people, or well-known characters, are the hilarious ones... if you possess the "cultural literacy" to recognize them!
We have the "Shrek" character from the animated film, green in color with trumpet-like ears. That's all we need to know who it is. "Hulk Hogan" has a t-shirt bearing his name, partly obscured, which is the closest we get to a "label" on any of these photos. His moustache, thinning blond hair, and characteristic hostile expression are sufficient to identify him. With "Gene Simmons" of the rock group KISS, it's the make-up and, again, the facial expression that create the identity.
The Adolf Hitler caricature is particularly interesting, illustrating the universality of Nazi era imagery without overtly indicating Naziism via armbands and swastikas. A moustache and hairstyle, along with a raised fist, presented in black & white, are sufficient to identify the character.
Finally, who would have thought that a baseball cap, wire-rimmed eyeglasses, and unshaven round face would become so well recognized as to become a required part of our "cultural literacy"? How many film directors are identifiable by sight in the first place? Yet here we have, clearly identifiable despite the lack of labels or nametags, an instantly recognizable caricature of Michael Moore.
"Cultural literacy." It's the stuff you gotta know in order to understand the world around you. And to get the jokes on "The Simpsons."
Here's a reference for you:
Hirsch, E. D., Jr., Kett, J. F. & Trefil, J. (2002).
The New Dictionary of Cultural Literacy.
New York: Houghton Mifflin Company.