Why can't I attach images in my forum posts?
Couple reasons:
#1 it bloats the forum database, making nightly backups very intensive
#2 The forums are intended to be forums, not photo storeage. There are services, both free and paid that handle that.
People where posting attachements in all of the forums, and it was a nightmare. You are allowed to post images only in the Jeep Picture showcase.... and even then, don't even think about "hotlinking" them from there.

Besides, we prune that forum at 30 days to keep the database size manageable.
Now, you can utilize your ISP space, or a photohosting service etc... and still have your pictures in a post simply by using the IMG button, and inserting the URL of your image.