Creative Commons licensing is a wonderful idea, allowing an artist to set the terms on which their work can be used. The image below is one of mine from Flickr but based on another photograph, (licensed under these terms) so allowing me to create derivative works and in this case even to make commercial use.
Having access in this way is really stimulating, letting me work with a host of images I would never have taken myself and allowing me both to practice digiatal manipulation and in some cases to produce works that stand on their own.
This one for example is I think different enough from its source to have a standing of its own. I'm looking at how I might post some of my own images under a Creative Commons license. It seems reasonable that if I make use of these I should also offer some up.