Official Google Blog: Sort by subject in Google Images: "When you’re searching for images, sometimes it can be hard to come up with exactly the right words to describe what you have in mind. For example, when you think of London, you might picture the iconic clock tower or the big Ferris wheel. You may not always remember the names of those landmarks, but you can visualize them in your mind. To make it easier for you to find images in situations like these, you can now use Google Images with sorting.
When you search for [london], by default you’ll see image results ranked by relevance. Click on “Sort by subject” in the left-hand panel and you’ll see images organized into categories that will narrow down your search and help you find the exact image of London that you want."