Use the tags to insert the tumbnail of your image into the post, and then wrap that in the tags to link back to the full sized image. Of course the options change based on the forum. Cnet, for instance, doesn't support the posting of images in posts, only links to them.
One of the forums I like to post pictures on does not allow html code (which I am quite familiar with).
Could someone give me the code for posting a clickable thumbnail? It would involve 2 different pictures. the 1st a 100 x 100 pixel (or custom small size) thumbnail and when clicked would lead to a 2nd larger image of the same thing or possibly another full-size photo. All my pictures are stored on a hosting site and have URLs.