There's two ways. In your message window there's a yellow box with mountain button, click it and just drop the URL in there and the site will do it for you.
If you're using something like Photobucket, there's an
code for forums, you'll want to use that but you don't have to use the picture button on the message window. Just copy that, and paste it straight from Photobucket.
Hope that helped!

If you're using something like Photobucket, there's an

Hope that helped!