The easiest way is to upload the image you want to use to a website somewhere, like photobucket, and then enclose the address of the image in IMG tags in the Signature section of your profile, which you can find in Your Control Panel/Edit Signature (You can get to your control panel by using the messages link at the top of each page - it'll probably read "0 new messages").
It should look a little something like this:
[*IMG]http:///www.photobucket.com/myAccount/MyPicture.jpg[/IMG*], but without the *'s and with the address of the image you want to display.
Please bear in mind that there are some size restrictions, as explained
here.