Are you familiar with UBB Code? The image tag is {img} so to have an image appear you would type {img}http://www.yourdomain.com/image.jpg{/img} Note, replace the {} with [] when actually using the code.
If you're not familiar with UBB Codes, use your board's faq link which will explain them in detail. There is also a link to the left of the message window on your board that should say UBB Code is enabled (which you can click for UBB Code examples and uses).
