You first need to get them on a web server (or image hosting service) somewhere. Then you put UBB Code (see the note at the left of your reply form) to refer to the URL of that image. If I put in the UBB Code...
(img)http://it2.evaluand.com/gti/images/firesys1.jpg(/img)
You'll see that image on the board. (Note that I didn't use square brackets in the example, because you would have then actually seen the image.)
K
EDIT - the new Firefox browser has a feature that is really useful in this context: The right click, "copy image location" trick.
[This message has been edited by Knestis (edited January 15, 2005).]