There is an interesting eye tracking story over on the Google Blog, covering their results from some usability studies they carried out.
Specifically: eye tracking to test the use of thumbnails in search results. Though the blog report is fairly brief, they did come to specific conclusion.
Our studies showed that the thumbnails did not strongly affect the order of scanning the results and seemed to make it easier for the participants to find the result they wanted.
There is a real time eye tracking video and some heat map images in the article too.
— Story from Official Google Blog.