Go to forum,forum profile information, go to the signature block and put it in using img blocks:[img]url for your picture[/img}, except make the } a ] at the end also. Unfortunately, the powers that be have lost control of our ability to input pictures into the site directly, or to input items into the gallery even, so we can't even host our own pictures, unless it is already input...you can still use those.
Example... my horse girl would go into my signature block as:
[img]http://wwwtemp.texasxriders.com/d/35047-2/hg2.JPG[/img}
if I put a ] at the end instead of the }, the picture will show rather than the typed example. Use the ] at the end of the img string.