Go to imageshack.us or tinypic.com or any other image hosting website and upload images you want to use. The sites will provide you with a link. They generally have forum links, but they all have direct links.
Take the link and place it inside of the IMG tags, which look like this::
[img]your_image_url_here[/img]
To edit your signature, click on the PROFILE link at the top of the page, then in the left sidebar navigation click on FORUM PROFILE INFORMATION, then scroll down and look for the SIGNATURE BOX. You can place you images and text there. Just use the IMG tags like I said.