The Getty Iris Blog is the Getty museum's blog about its art collections, research, conservation and digitized open content.
HN links it for announcements of open-access image releases, and commenters cite it for conservation before-and-after photos.
Here's a link to the actual collection: http://search.getty.edu/gateway/search?q=&cat=highlight&f=%22Open+Content+Images%22&rows=10&srt=a&dir=s&pg=1
Getty's blog post about it: http://blogs.getty.edu/iris/open-content-an-idea-whose-time-has-come/
An example page where you can download a 4264px × 3282px copy of Van Gogh's 'Irises' (28mb .jpg): http://search.getty.edu/museum/records/musobject?objectid=947
I'm wondering how long it will be before someone creates a .torrent with the entire collection...
In practice yes, Museums occasionally clean & restore their paintings which shifts the color balance, grain, texture similar ways.
There's a some great pre/post conservation photo in this article: http://blogs.getty.edu/iris/as-layers-of-old-varnish-are-removed-italian-renaissance-painter-ercole-de-robertis-brilliance-is-once-again-revealed/
Getty Archive Source Post: http://blogs.getty.edu/iris/77000-images-of-tapestries-and-italian-monuments-join-open-content-program/