ok I ll give you an example of how to type it with my banner.
[IMG]http://img2.ranchoweb.com/images/scorpionx/wolvebanner.jpg[/IMG
If you if you add a ] to [/IMG at the end of the example above and make it [/IMG] it should turn out like this
EDIT: I looked at the url of box with the red X in your sig and there is no file extension(.gif,.jpg,.jpeg) make sure you have the correct file extension included in the url.