Well I was trying to give them a reason, to get new users to explore and understand all of the forum's features so they know how to do stuff like post an image.
I guess, but its mostly going to promote achievement whoring more than anything. You should use the forums becuase it's a fun place to relax and joke around, not so you can earn an achievement. And if you want them to learn how to do it, just tell them they have to use the forums occasionally to be considered an active member, I mean this is the first place any important information gets put.