I spent a ton of time working with the Met's database to create this:
http://art-slideshow.net
They only made the public domain images downloadable + zoomable, about 204,000 (not sure where you got the 50k figure). Stuff from 1900+ is often still under copyright, so that's why most of them are low-res.
Of the 204k, I filtered out baseball cards and cigarette packages, leaving 193,525. Of these, 151,782 are what I considered high quality: 1800+ pixels width and/or height.
There are def tiny images in there like this...
http://images.metmuseum.org/CRDImages/ao/original/VS91_37_16...
...but really anything interesting to an art student has hella resolution.
All in all it's a pretty traditional, conservative collection -- Google's is great for newer and crazier stuff.