Another change is that the image results group has 4 images instead of only 3 and it can be displayed inside the list of search results, not just at the top or at the bottom. When the image results group shows up at the top of the page, the thumbnails are significantly larger.
Building an universal search engine that manages to rank web pages, images, videos, books, maps turned out to be a difficult task, so Google tries to do something easier: rank OneBoxes for different specialized search engines and display them in appropriate places. You can find a version of Google that doesn't include universal search results at searchmash.com.
