Google is best, probably because of all the other bits and pieces it has (GMail, Google Reader etc.), but Bing image search is nice, in that it doesn't need you to go to next page EVER, it just keeps adding pictures at bottom of page. Also, images get bigger on mouse over, and if you click an image, image results stay at side while the page is loaded, so you can click on next image without having to go back to search results. Still Google is better.
