Which forum is that? Basically you need to add the image code to the url of the picture like this:
Just change the brackets into square brackets. I have to check that all the forums supporty this. I think I set all of them, but I might have missed one.
By the way, it does mean that you need to have the picture uploaded on the internet somewhere. You could use the url of picture in the cat pages for that or most urls of pictures on the net. To find out a picture's url, right-click on the picture in your browser and select properties. There just copy the URL and paste it here (don't forget to add the img tags).
Hope this helps. Let me know if you need any more help in the matter. I'd love to see your pics!