The Google Art Institute has created the Art project in partnership with Museums from all over the world to enable access to thousands of works of art from different time periods.
You can also use the site creativecommons.org to find images licensed for reuse by the creators of the photographs.
And finally, check out Encyclopedia Britannica for articles on different artists. Citations for the articles are available at the top of the first pages.
For contemporary mural painters in our midst, see this article from Oregon Live: International Mural Artists collaborate publicly in Portland (photos)
This panoramic view of the Sistine Chapel mural paintings by Michael Angelo is so vivid that one has the impression of actually being inside the Chapel.