Wow, very broad question!
I'd suggest expanding your knowledge of the 4 PSA/DNA photo types by picking up a copy of A Portrait of Baseball Photography by Henry Yee and Marshall Fogel, who do the photo authentication for PSA. Besides explaining the photo Type system used by PSA in more detail, it also gives brief background info on a number of news services and photographers. It won't answer all of your questions, but may help you home in on more specific information you're seeking.
As "rating," value, and popular subjects, I think you will find photos are far more subjective than most other areas of sports collectibles. There is no price guide, and no widely accepted grading system. It's all about the content and originality, and though popular subjects mirror other areas of collectibles, all photos of a given player are not created equal.
Basically, I think you will find that the further you go down the rabbit hole, the more you will realize how impossible it is to address any of your original questions in a succinct manner. As I read each of them, I think to myself, "Holy crap, where do I start."
