Also, just because a picture does NOT have a watermark does not make it available for use. Whether there is a watermark or not, it is still property of the owner.
The one thing missing is a mention of Fair Use Doctrine. In some very limited cases (news reporting, commentary, education, research) you can use a picture with a copyright. However, before you do, you should research Fair Use.
I'm amazed at the number of times photos are used on websites and the webmaster does not understand copyright laws.
Yes, this should be a sticky.