are you asking for sites? mmm there are many. one of the best tool is obviously google image search. if you want any other sites just go and search in google or other search engines with the search terms like free picture downloads, free image downloads etc.
You could try flickr. Use the advanced search function to find images available under the creative commons licence, and that can be used commercially. That's how I source many images.
did you mean sites? if yes, i personally recommend flickr.com, the have great stocks and varieties of images