Credit and citation, these things have always been important, but with the age of information, it's become phenomenally easy to use an image without the owner's permission without even realizing that the image had an owner to ask. Well, sometimes. Sometimes it's fairly obvious. Artwork and photography belong to the artist, that much should be obvious. Nice, safe stock photos provide a way to step far away from artist toes, but good stock photos can be hard to find without a provider. Dreamstime is such a provider, with an impressive collection of 38 million photos, and they offer plenty of free downloads as well.