It's a little different than posting a pic in a thread in that you have to have the pic hosted somewhere on the internet. You can do that easily by posting the picture in the test section. Here's what you do...
1. Create a file (or you could use an existing .jpg pic) and post it in the Test section of the forum.
2. Go to the pic that was just uploaded in the Test Section and right click on the mouse while hovering over the pic.
3. Choose "Properties" out of the list of options
4. Highlight the Location url and right click and choose "Copy"
5. Go to your profile in "Forum Profile Information"
6. Scroll down to the "Signature" box and put your cursor in there then right click and choose "Paste".
7. You will need to put these commands (WITHOUT the spaces) in the front and rear of the url you just pasted into the signature box: [ img ] [ /img ]
As an example it would look like the string below WITHOUT any spaces. If I left out the spaces here in our example, the picture would show up.
[ img ]
http://www.cuda-challenger.com/cc/index.php?action=dlattach;topic=20523.0;attach=40507;image [ /img ]
If you have any problems or questions, just let me know.