Are you actually grapping the image from a remote location to store permanently on your size, or are you just referencing the image (like is often the case in the banner ads).
The former situation is complex, the later is simple (add <IMG SRC="http://somedomain.com/image.jpg" HEIGHT=xxx WIDTH=XXX>) but that is not truely resizing the image, just the way the browser renders the image. I doubt that is what you are really asking. Drop me a line with more details and I will try to help.
-Brandon