Wikipedia? http://upload.wikimedia.org/wikipedia/commons/a/a8/Van_Gogh_...

If you're into Japanese art check out John Resig's Ukiyo-e, some images have pretty high resolution: http://ukiyo-e.org/

Not a unified one, however most museums have decent interfaces to their collections these days. I would highly recommend the Met Museum in New York's catalog, for instance. Some produce limited resolutions or force you to decode some kind of image-slicing, but in general you can get super high resolution copies for free that would be suitable for personal use.

And if you want an api to the met museums art... I made http://scrapi.org . Full disclosure: I work there.

Maybe this: http://www.fromoldbooks.org/ ? Might not be as high resolution as you'd like but still an interesting collection of art from old books.

gallerix.ru has a lot of relatively hires art: http://gallerix.ru/album/Museums

piratebay?

Google images is great for that. Search by keywords, then set the minimum resolution under "Search Tools". 8 megapixels should be enough for a good print. 20 megapixels - for huge ones.

Internet is still kind of free. Just don't tell anyone that you're doing this.