You can edit posts for spelling mistakes, fixing broken links etc. for twenty minutes after the initial posting. Aside from that, there really isn't any reason why you would need to go back and change what you previously posted. It's fairly standard procedure on forums around the web and if it IS something you feel is important, simply contact a staff member to assist you as they can always edit any post on site.

If all you want to do is add photos to your thread, then simply make a new reply in that thread which will also bump the thread allowing everyone to see the newly posted, updated photos.