whoisgregg - 2:44 pm on Feb 1, 2008 (gmt 0)

Is it like a mini-screenshot of the page?

Yes, however it's actually a screenshot of the portion of the page the user was zoomed in on at the time they add it. In other words, if you are zoomed out, the button looks like the entire page, or you can zoom in to the logo (for example) and then set the bookmark and the icon is the logo (plus whatever parts of the page were still visible around the logo).