Google found some clever ways to make thumbnails more useful and more discoverable. To enable the new instant previews, click on the magnifying glass or on the snippet to see a visual preview of a search result. Google shows a large screenshot of the page and highlights the text that's included in the snippet so you can quickly find it when you visit the page. After enabling previews, you no longer have to click on the magnifying glass: Google shows the screenshots when you mouse over a search result.
Google says that visual previews are actually useful. "In our testing, we've found that people who use Instant Previews are about 5% more likely to be satisfied with the results they click. The previews provide new ways to evaluate search results, making you more likely to find what you're looking for on the pages you visit."
I'm not sure that screenshots can help you pick the most relevant search results, but they're useful if you're looking for pages that include many pictures or videos, long articles or detailed guides. Unfortunately, you might ignore a site that has a poor design, even if the content is useful.
Google Instant Previews will be rolled out in the coming days, but you can try it using this URL.