Everyone should of course get all of their images from Jon Arnold Images, but if you're after something a little cheaper, then iStockPhoto is a good place, but if you're after something even cheaper, then Pixel Perfect Digital do stuff for free. Read the terms and conditions on all sites before using anything though.
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Another site with free images is Damon's photo gallery. Pictures are of varying quality because they've been taken by amateur photographers but they are free to use, the search engine is good and you have a wide variety of subjects. You can also find audio and video files.