Are you testing the value or user demand for that feature? I agree with the GP. Screenshots reduce information density, increase bandwidth (load time), and for me take even longer to visually parse than a text result. That is, they interfere with what I consider to be the most important things I need from a search engine. Having screenshots populate over time as they load moves my eyes around the page and acts as a distraction--at least for me.

Google tested a similar "view a screenshot of the search result" feature a while back. You'd hover over a result and it'd show the screenshot in a column to the right. For whatever it's worth, they axed it, although I don't know the reasons why.