The White House has released many photographs of President Obama under a very liberal license. I had heard that the creator of the iconic Obama posters used during the election got in trouble with the Associated Press for allegedly using an AP photo. If only the artist were to have had available a site like this.
Even though one link on the page says that there is a liberal license on the page, another statement on the page says the photos are not to be used for any derivative purposes. A case of battling forms.