is it possible to set posts per page not by post numbers but length ? because sometimes my forums have 6 pictures in one post and 10 posts like this so i have 50 pictures per page and sometimes i have 10 posts with no pictures it really looks bad.
There's not really a good way to do it, the only logical way would probably be to add a counter for the number of pictures to the messages list and select by that, but anything will be a completely custom production for your site, which I suspect you will have to pay for it to be developed.