Is one photo/wallpaper/screencap/icon worth as much as one link?

No one (*) knows how the different content is weighted, though I agree with your sentiment.

Other content is often "worth" more, as far as the depth of the information provided for the club's topic, but images are the most likely item to get a rating of any type of content. This is because it takes the least time to evaluate an image - you don't have to read anything or watch anything like a video or a link. I think this latter is the main reason that heavy image posting seems to get "rewarded" with more medals than a similarly heavy posting of other content: they're just getting rated more.

* Aside from the Fanpop staff, presumably.