It takes brains to decide which photos to post. Consider @thestickupboys who post cool gifs they made for millions of fans. Those are definitely worth an upvote.
Some pictures speak a story and some pictures make melaugh. Some pictures are worth a thousand words and some are not. I don't know exactly what algorythm to use, but It should be possible to judge objectively whether a picture is spam or content.
It is definitely possible to differentiate! And I believe that having this space for posts with more photographs would make POB just become more inclusive.