New Tip: Companies like iStockphoto got wise. Now their preview photos come with a large white border at the bottom i.e. the preview photo and the photo you buy are not the same. In fact the white border is calculated to be big enough to make the image register as a different image to Google image search. Their site then just uses CSS to not display the bottom part of the image. If you want to find their images you need to download that preview image and crop it manually before doing an image search.
Lots of advice, and quite a bit of paranoia in this thread. As I stated before the key is to know where the image came from. High end companies like Getty license images for a particular page, so can easily find violations, low end companies just sell images and do not track them.
]
