If you go to get.google.com/crowdsource/imagelabeler/category, you'll see this message: "Ready to help Google Image Labeler? Look at a few public images to see if Google is organizing them right."
Then you can pick a category like birds, cats, dancing, concerts, food, cars, mountains, sky. If you use this URL: get.google.com/crowdsource/imagelabeler?label_str=Dogs, you can add your own category (replace "Dogs" in the address bar with something different like Rainbows, Stairs or Moon).
Google shows Creative Commons images from Flickr categorized by Google's algorithms. You only need to answer to questions like "Does this image contain dancing?".