First get a host..or some free web space
..Typin in free web space at Yahoo or w/e..
When u've found one.. upload ur picture & u shud have a url/adress..
Somewhat similar to this:
www.whatever.com/shorty/image.jpg <- example..

Then take that adress.. & go to ur User CP and click signature..
..Paste that adress in there BUT..
Put it between [*IMG]..adress here..[/*IMG]

without the stars.