I wrote an article for the PHSNE Journal last year about image collections that are accessible to a traveler in the New England area. I focused mainly on those places one could actually visit to see work that is on the wall, though clearly the bulk of most collections these days is more easily seen on line. Here is a link to that article.
I welcome additions to the list through comments here or emails that prompt me to add to the original list. Another list, of great on-line sites for images will be coming. Again, I welcome comments and additions.