Google has long included image results in general searches. Over the next day, it's tweaking the way those results are shown, with the best-matching images appearing larger in results.
Google's blog explains the concept pretty simply: "When we're confident that we have great image results, we'll now show a larger image and additional smaller images alongside." According to Google, the change will appear over the next 24 hours; it doesn't seem to have hit the company's Australian servers as I write this.
Show me the pictures: better format for image results [Official Google Blog]