keyplyr - 1:33 am on Feb 4, 2013 (gmt 0)
Snapshot, thumbnail, whatever you call it so long as it's cached.
The point is IMO, you don't need to allow them indexing privileges to your image files for them to get a snapshot/preview of your site any longer... at least not from the tests I did last year and I see no problems to date.
I just blocked all direct (no referrer) requests besides some known bots.
Then you are blocking a huge amount of valid users. Almost all browsers can turn off referrers nowadays, plus the list I gave in one of my posts above.