I don't know if this is common knowledge but I figure I would post my assumption/findings.
I run a ecommerce site and each product has well described image file names. I noticed in Google image SERPs the thumbnails for other products came up when searching for specific products. (because on the bottom of my website has a 'featured' list with several other products w/images) The solution was to simply block the thumbnail folder in robots.txt. A few weeks later, after the thumbs dropped from the image SERPs, the traffic referrals from Google image search doubled. =)
I am not 100% positive if this is coincidence or cause and effect. My assumption is the latter.
In Google webmasters, image impressions have doubled and CTR has increased (by how much, I am not sure).
Would anyone care to comment on this and either support or poke holes in my assumption? (the site has been live since Jan 2011. And the majority of product images shown no impressions in Google Webmasters before the block)