lol, you forgot step one
first, find a place to upload your image to. if you are cheap / dont have access to an image host - you can use geocities, so long as you name it a .txt instead of a .jpg. they wont allow remote loading of .jpgs.
once you have that, make sure that you know the url, and then do as belgarath has stated. some forums do not allow images to be linked, but this one does (living room).
if you are still unsure, check out threads where people do have images linked. lemme
is linked from my account with my school.