Assume that they are all copyrighted, unless you can get written notice from the creator of the work, that he has allowed it to be used without license.
If you find a photo, and can locate the owner, contact them, and see if they will allow a limited use license, for a specific purpose. As long as you dont sell it, you might be allowed to use it, if you give proper credit to the creator of the piece.
And yeah, I copied that from yahoo answers.