Right click on the picture and save it to your computer. Then upload it to a photosharing site (I use Photobucket.com)
Then, up at the top of the forum, click on the Profile tab. To the left, click on the "Forum Profile Information" link and scroll down to the bottom of the page. There's a box that says "Signature." Enter the URL where your picture is online with [ img ] at the beginning and [ /img ] at the end and click "Change Profile" at the bottom of the page.
Your code for your signature should look something like this: [ IMG ] http://img.photobucket.com/albums/v358/TiciaT/ehellsig.png
[ /img ]
But without the spaces.