Images in the public domain are fantastic things for both digital artists and traditional photographers and that's where sites like The Public Domain come in.
As a photographer, it's interesting to study what people have been doing in the past. How they have framed their images, what poses they used, what they bothered taking photographs of and thinking about how that can improve your own skill levels. For artists, it gives you something to work from and also becomes a source of ideas.
I am also a big fan of writers using pictures to inspire their work. Here are two images from the The Public Domain which are just wonderful. I keep trying to make up stories that go with the pictures and thinking about different things I could create using them :-)