Hello (and sorry for bad english),
I really appreciated the open access policy for digital images of the Washington's National Gallery of Art: we can freely use theme, even for commercial use.
The British Museum also launched a similar project but for commercial use images aren't free.
Do you know some other history museums that launched a digital images database?
Smithsonian SIRUS - http://sirismm.si.edu/siris/sirisimagegallery.htm
The Getty - http://www.getty.edu/art/collection/
New York Public Library - http://digitalcollections.nypl.org/
The Walter's Art Museum in Baltimore has an open-access policy for non-commercial and scholarly use. Specifically, all their images are under Creative Commons licenses. http://thewalters.org/rights-reproductions.aspx
Not a history museum but the US Library of Congress allows you to search it's collection online. I don't know how much is digitized out of the whole collection but I was able to find quite a lot of old photographs of my hometown in pretty high resolution image files.