First, you have to upload it to an image hosting site, i.e. www.imageshack.com
Then, you have to take the link, which should end in .jpg, and click "Settings" at the left side on the forums. Scroll down to "Signature", and paste the link in the box to the right. Now, highlight said link, and click on the "Insert Image" tag below that. This should add (img)Imagelinkhere.jpg(/img) to your signature! Except the brackets should really be square brackets.