From a bandwidth perspective, I would say to just limit the file size on attachments.
Unless it is important to you to provide that capability, because over time even small attachments can use more and more bandwidth (so do avatars for that matter, until they are cached).
Is it possible to limit a specific forum to different file attachment sizes? Thus, set up a specific forum that is more suited to having pictures, not necessarily porn.
Or just a photo album area where you can just remove images that are too large, or gets overpopulated with porn that really isn't totally necessary to the function of the board.
You can just set a rule or guideline that states porn should be linked off-site, just for ease of use and bandwidth. People can put attachments up for that stuff if they want, but if it is too graphic (your post voting system I suppose) then have it linked off-site. This could self-heal itself via the voting system. This way, you don't stop any free expression, just possibly more work-safe or just being courteous.
Or, can you also have voting for a specific post instead of an individual+post combination (which is how I understand it works)?
Or, vote on at attachment?
I don't know the complexity of the system. Can the person who posts the picture put a content level on it? i.e. 1 is G rated 5 is XXX and you can as an individual set your rating level so you don't see pictures you don't want to? Then, an admin can set the content level of the photo so it doesn't get seen by those that don't want to see it if it goes outside a specific content level?
That would keep bandwidth down after a while, and if people could vote on content level, then the people who filter won't see what most people would think of as porn I suppose just from individuals voting, thus less admin intervention (as the board grows).
Leave it to me to come up with something complex.
-Scott