If it's on a website like photobucket or myspace, use this code:
If not, there's a small box under where you type your message which says "download image shack toolbar" on top of it. Click browse, find your picture, and then click "host it". Then use the same code that I gave you (or the code that they provide that looks like the code I gave you), and copy and paste it into the message you want to post.
Hope that helps!