I think it depends what your looking for. Yahoo and Google are obviously the main ones, I'll normally use Yahoo for pictures because in their images search they use Flickr, which is a brilliant picture site. For pictures ill also use
www.picsearch.com, thats quite useful. Usually I find googles picture thingy a bit shit, better to look for information on though than yahoo. I've never heard of Bing before, shall be trying it out =].