You need to have your image hosted (
www.imageshack.us for instance), then copy the link, click the button "Profile", "Forum Profile Information" on your left side, up there, scroll down to the Signature-field, paste the link and add

behind the link.
Example:
[img]http://www.planeshift.it/graphics/pslogotop2.gif[/img]o/
EDIT:
Sorry, just do it according to the example in the Code-field, since the tags wont show up in the text above.