First you need to get host for your picture. Try
Boomspeed Then after you have done that. Go to your user cp, then Edit Profile, then scroll down to the signature box and put in [img*]yoursighost[img/] (without the stars). And your image should appear beneath your name.
For example, mine is [*img]http://members.lycos.co.uk/jaajoe/hpbimg/metroidsig.jpg[/img]
(Without the star).