An article in the Times on changing museum policies and downloading.
The Rijksmuseum

“We’re a public institution, and so the art and objects we have are, in a way, everyone’s property,” Mr. Dibbits said in an interview. “With the Internet, it’s so difficult to control your copyright or use of images that we decided we’d rather people use a very good high-resolution image of the ‘Milkmaid’ from the Rijksmuseum rather than using a very bad reproduction,” he said, referring to that Vermeer painting from around 1660.

It's not mentioned in the article, but the Met now allows downloads, not high quality but enough for the web. The original file of the Hals I posted recently is 3.1mb.
When you go to the page for a painting, click on "full screen" and a download link appears on the lower right.