Like on iwebtool.com - when you hover over the sponsored links at the bottom, a preview window comes up. I understand how to move a div element around with the mouse and stuff, but I can't work out how they generate the preview images.
Ah. There are a couple of ways. One is to manually create a snapshot of the page in question ("Print Screen" on your keyboard, then open your graphics program and paste it in there and save). The other is to create a program that will crawl the site and take a snapshot of it. Unfortunately THAT is way above my skill level.
I just found http://www.thumbshots.org/ - I guess syou use their API? Ah well, I answered my own question.