How do I know whether a picture is copyright protected?
It if was created by someone and published anywhere, then it is copyrighted.
Copyright protection in the US and most countries is automatic as soon as a work is shown to anyone other than the creator.
Pictures belong to the photographer. Unless he is selling them or signs over the rights to you, they should be considered copyright protected.
The copyright may have run out if it was published before 1923.
If it doesn't say that you can use the picture royalty-free, then you can't use it without permission.
Almost all pictures are, unless they are 'shared' (as in Webshots - and even then there is some measure of copyright protection in that someone who did not take the photo cannot claim credit for it). Even public domain images - i.e. images taken prior to 1923 and conflict images - are copyright-protected to a degree. As someone mentioned before, photographs are usually the property of the photographer or his/her heirs, but can be property of a country (certain photographs taken by war correspondents on a 'grace and favor' basis), or others to which photographers assign rights (good examples are photographs taken for magazine and newspaper articles or record album covers, which belong to the publication/record company).