My idea is to create a simple photo gallery IN the board.
For example, in one category users can post images instead of topics.
What do you think about this?
Feel free to create a photo gallery mod. I'm certain this will not ever become a core feature of SMF. Perhaps you should post your request on the mods request board.
Maybe take a look through this thread.
http://www.simplemachines.org/community/index.php?topic=40113.0