Site thumbnailing service that lets you cache thumbs?
 

I'm talking about services which load and render a site, then generate a thumbnail as a visual representation of what it looks like. The thumb is accessed via an API.

Last time I looked at thumbshots.com (which seems to be down at the moment) they didn't allow caching... so if you have a static page with 50 shots and 1,000 people load it you pay for 50,000 thumbs.

Are there any services that allow caching of the thumbs they return, IOW a once-off fee for each unique site thumb?