If the forum supports HTML, simply code in the appropriate HTML tags for images (img src=). Be sure that the image is located whereever you reference the code.
If HTML isn't allowed, you can always use the img markup language available through these boards. Click on the "Markup in your posts" link when making a reply for information. Basically, the image markup allows the same thing as HTML. But instead you simply reference the location of the image, without having to do any other coding.
The boards here, from what I've seen, allow both HTML and markup. The forums I run only allow HTML on some, and markup on all. Quite easy to do.
Danbo
http://danbo.comhttp://danbo.com/forums/wwwthreads.pl