How do I view other people's screenshots on Steam?
To view your friend's screenshots, your friend must first have saved said screenshots (obviously).
Firstly, you must view their Steam profile.
Secondly, you must click Screenshots on their profile page. It looks like this:
It should then bring up a list of all of their screenshots for your viewing pleasure. It will look something like this:
This all depends on whether or not your friend has his or her screenshots set as Public, Friends Only, or Private. If you know for a fact your friend has some screenshots, but you can't see them, the chances are they are private.