Google - King of the web crawler, far outpacing the others in pure numbers but not necessarily quality; they’re also king of the lo-res Wikipedia image.
Bing – So far, lower number of results compared with Google, but relevancy and quality seem better.
Pinterest – Since Pinterest doesn’t display tag words, its search seems to be returning results from any number of sources: title, description, tags. Results of same term, different day will vary widely.
Instagram - People love to take self portraits with their phones. A lot. Results of same term, different day will vary widely.
Flickr – When filtering via the “interesting” option, the first page of results a exactly that. By far the most artistic and thoughtful of the bunch and usually the smallest results pool.