I’m having luck with the Creative Commons Image Search. You type your search term into the box, then select one of thirteen sources for images, sounds, videos, and written content. Based on my extremely thorough research (which consisted of typing “corgi” into the search box, playing with the permissions, selecting a source, and cooing) it seems that Flickr and Wikimedia Commons have the best quality photos. Flickr perhaps requires a little more patience, though. Since it’s a social sharing site, you tend to get rows and rows of similar photos from people dumping out the contents of their SD card. However, there are some gems from amateur photographers that make it worth sifting through the search results.