I don't use a Mac, lavlee -- but a friend of mine who does uses a program called "Fetch" I think.To post an image here or elsewhere on the net, it first has to have a URL. You need to have the image saved on your computer. Get a free account at one of the webhosts -- or, many people get at least 25mb of free space thru their ISP (such as Yahoo), I think I have even more than that thru Comcast. You need to upload your image to your free web space using a FTP (Fetch -- download it at apple.com ). Once you've uploaded it, you'll have a URL for the image at your free web space.
Then you can use the BB code to make it display in a forum post. All you do is put "img tags" around your URL in your reply-text-box, so that it looks like this, but without the spaces inside the image tags:
[img] www.wherever.com/lavlee/lavlees-image.gif [/img]