Awesome, right? Well, on the toolbar (Look up from the Post Visitor Message box on your left of toolbar) there is a picture like this:
You click on that and there is a box that pops up that says: "Please enter the URL of your image:". Look at the bottom and there is a box and you paste or enter the URL of your picture and click OK. There will be a URL that pops up in your visitor message box. It says [IMG] (The URL of your pic) then [IMG] again. Don't worry about it, when you click Post Message the picture will appear in place of the URL.