Following up to what I'd stated earilier in this thread regarding getting the thumbnails to show up in Google Custom Search, After I added the <meta name="thumbnail" content={url to thumbnail}> to my site header I'm getting nearly 100% correct thumbnails on every search hit.  You'll need your own code to generate the url to the thumbnail image.

Without adding this meta tag I was seeing only about 30-60% correct thumbnails or no thumbnail used.

This is documented here in the CSE docs: http://support.google.com/customsearch/bin/answer.py?hl=en&answer=1626955