Large screen shots prompted a huge negative reaction about two years ago in this WebmasterWorld discussion about SearchMe, a new engine with a very attractive interface, where the size of the screen shots was considered by many to be a threat to click-throughs.... [webmasterworld.com...]
While members discussed blocking SearchMe or at least not allowing it to archive content, that solution is more problematic with Google if large screen shots are implemented. Google drives a lot more traffic than SearchMe did. Site owners might be in a very difficult position if they're the only listing on a serp without a page preview.
A lot of course depends on the particulars of your site and page, and on the nature of your business model.