Co-maintainer of PagePeeker Screenshots Drupal module since .
The module uses the PagePeeker.com service to display thumbnails and favicons on web pages.
It works particularly smoothly with the Link module: simply go to the display management page and select the "Pagepeeker thumbnail" format. Same formatter works in Views as well. There is an additional option to choose the screenshot image size.
There are free, link-back and paid versions of the service available, and all of them are supported by this module. For more info, see http://pagepeeker.com/free-thumbnails/ and http://pagepeeker.com/site-thumbs-pricing/.