I almost exclusively use Wikimedia Commons. However, I write on a lot of old historical subjects, so I am almost always using public domain pictures. Wikimedia does have a lot of more modern files that can be used under the Creative Commons license, but I imagine there are some other sites that are preferred for this sort of thing.
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I cried, "Mea culpa! Mea culpa! Mea culpa!"; resolving to turn over a new leaf, but having looked at the suggestions here (with the exception of Marian Designs's: http://karenswhimsy.com/public-domain-images/) I'll continue to tread the primrose path