Of course, in a very broad sense, everything is nature. To quote a fab tune from the early 90's (I think!):
"You can not go against nature / because when you do / to go against nature / is part of nature too." (Crooned by David J. Haskins, of my beloved fave group Love and Rockets)
Of course, being aware of the photographic resource's ownership is essential in case that amazing art you create ends up being sold or published. Finding free-use images in the public domain is not that difficult. There are great sites out there featuring professional photographers' images -- just google your subject and the words "public domain photo." Here's and example. The USA.gov site has a good collection here. Another interesting resource for exploring images from nature is the U.S. Fish and Wildlife Service National Digital Library There are some very nice old images on the site taken from painted glass slides, including this groovy bird.
What are YOU going to create today?