Let me get this straight... you don't want to upload the image to a web server, but just leave it on your hard drive, and link to it on the forum? Short answer: you can't. Long answer: what the hell kind of question is that???
You do it by uploading it to a sever which isn't lame like Xoom and Geoshitties.
Don't ask me where to get it.
Luck out and find a friend who runs a DSL server like I did.