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They make you pay a high price for those advantages though! If you're looking for cheaper sources then these two can be quite good for celebrity shots.
For web use their cheapest sizes are often more than adequate and they're much less costly than the big boys. It can be more difficult to find what you're after though, as there tend to be fewer good quality images to choose from and they are often not so up-to-the-minute.
These are all providers of 'Royalty Free' images. Just to clarify, for those not used to stock image jargon, Royalty Free means they are free from further licence fees for re-use (royalties). You do have to pay an initial one off charge but after that you can use as many times as you want for free. As a photographer I constantly wish that the industry had decided on a less confusing term for this form of licensing - preferably one without the word free in it
I don't know of anywhere offering truly 'Free' pictures of celebs, ie. with no fee at all. You'd have to be very careful with any sites offering those as you could get your pants sued off by said celebs if they haven't signed model releases allowing use of their images.
I'm sure others will chip in with better suggestions.