Flickr.com has a lot of photography that is under a Creative Commons license, meaning that, if you follow the terms of the license, you can use it without asking permission.
For public domain historical art, try
Wikimedia Commons; you can browse by category or do a search on a keyword. For m/m art, check the
Homoerotic Art Museum. Note that not all of the art at the latter site is public domain.
In the U.S., reproductions of two-dimensional public domain art can't be copyrighted, so if you find a reproduction online of a pre-1923 painting, chances are good that it can be used. Do note, though, that the copyright on artwork begins from the moment that it's first published, so if the work is created pre-1923 but is first published post-1923, it's still in copyright.