Looks like Amol discovered Snap Previews. I just don’t see what purpose they serve. A screenshot of a page will never help me decide if the content is interesting. For one thing, fifty percent of the thumbnail is generally useless logos and menus that tell me nothing of value.
The only thing worse than mouseover image popups are those horrendously annoying, double-underlined mouseover ad popups that so many sites are using now.