Not an automatic one...
Browse to the page, Alt/PrintScreen, open your favorite image tool (mine is IrfanView), paste, crop, resize, save as JPEG:
http://zephyr.ugent.be/TW05DBRH/docu...pDigDotNet.jpg
The site that I want to index mainly contains digital photos, so I use thumbnails of the photos, not of the web pages...