ios – Image Search API
Bing and Yahoo both have larger amounts of images than Flickr.
Flickr is more lenient in terms of what you can actually do with the images however. Both Bing and Yahoo limit what you can do. For example, Bing doesnt allow you to reorder your search results and Yahoo has a hard-limit on the number of requests you can make. Flickr allows you to do basically anything as long as you dont spam requests or abuse their users rights to the images.
Bing also tends to have the more detailed search engine, as you can search for different colors, and categories than Yahoo or Flickr.
I hope this helps, and good luck!