Don't feel bad, it took me a while to figure out how to do that. Let me see if I can get the forum to put up the how-to info without interpreting the tags...
Here's what you do:
First, you'll need to upload two different sized images, one 800 pixels wide and one larger, to a photo sharing site. Copy the URLs for both.
Code this line into your post on this forum. It may wrap around, that's okay.
HTML Code:
[url=http://web.address.of.your.high.resolution/shot.jpg][img]http://web.address.of.your.800.pixel/shot.jpg[/img][/url]
-Noel