The key is that the picture must already be on the web somewhere and being that most of you have your own website you can upload it to a directory there and then point the Forum to it and it will show up!
1. Start a new topic or begin to reply to somebody else's post.
2. Type in the question that you have or whatever you'd like to say about the image you're posting.
3. click on the Insert Image button above.
4. A dialog box will appear asking you for the complete URL of where the picture is (EXAMPLE - http://davidjay.net/forumpics/invite_sepia.jpg )

5. Then preview your message and make sure the sizing and text is correct and then post your image!
Let me know if you need any help!
cheers!