Do you have somewhere you are hosting it, or are you trying to upload it to the forum?
If you already have your sig hosted, use this:
HTML:
[IMG]http://your.url.here[/IMG]
If you're trying to upload it to the forums, there's a 'Browse' button under the edit area for your signature. Browse to your sig on your computer, select it, click 'Upload' on the sig edit page.
In your signature body, use this where you want your sig to appear: