Take the image and host it at
http://imageshack.us/. Take the link to image they give you there, and put in a set of
tags.
Ex. http://forum.esforces.com/image.php?u=15319&dateline=1121131810[img]
I omitted the slash in the final tag to show you what it should look like.
Thanks for asking. Some people try to post images straight from their HD rather than asking how to do it and they end up looking very foolish.