down at the bottom of the "post a message" window you'll see a button that says Manage Attachments. Click that, then browse your computer to put your picture in there. If the picture is too big you'll have to resize it in Photoshop or something like that.
The other way is to load them onto
www.photobucket.com - they'll give you 3 links to use under the picture. One of them will be [IMG}picture name{/IMG] and you just post that into the thread.