While trying to browse through things I can review, if I select a filter such as "Photography" and hit the filter button when I view a portfolio I'm unable to use the back button to go back to the listings because it was a POST.

Have you considered making the filter search work using AJAX, which would solve this problem and also make it a nicer user experience?

Edit: I made a portfolio and added a single photo to it, but now when I try to add another photo it says "You've reached your limit" even though it says I can add up to 4 more photos.