Well, a little off-topic, but none the less important.
If you don't have a computer not connected with your live website, i would recommend you use a 3. part service like http://www.shrinktheweb.com/
, since there is some serious security issues with reading in website pages which is required to take a thumbnail image of it.
I have coded my fair share of search engine and directories, and therefore also web crawlers, and one thing i have learned is a thumbnail capturing application is very likely to get infected with mallware, even if you trust the people behind the websites you create thumbnails for, even there websites can get hacked, and infect the computer running the thumbnail capturing application.
Anyway good luck with the thumbnail capturing.