You'll have to go into your User CP (user control panel) under edit profile, schroll down, you'll see a box for the signature pics/text.

You'll have to host the image somewhere (for example Photobucket or similar) Then you will have to use the image tags
[img] xxx (the url of your pic at the host) [/img] (removing my text and adding your url).

I have a feeling you might have to have a certain number of posts to earn the priviledge of a signature image here, but not entirely sure.