Google Art Project.
They have modified Google Street View to provide virtual "walking tours" of each museum, and when you see a work with a plus sign on it, you can see a very high resolution image. As only one example, here is Caravaggio's Medusa at the Uffizi in Florence. You can zoom in to reveal the rich details of Caravaggio's painting, but in the right hand column, you can click on additional notes, tags, and so on. If you click on "More works by this artist" you can go to the other works of Caravaggio in this project, at the Uffizi and in the other museums.
Sometimes, you can experience even more. At the National Gallery, you can see Holbein's "The Ambassadors" and in the right hand column, you will see "Media". When you click on that, you will see videos about the work of art.
To get started, please look at the Visitor Guide.
The AUR Library has included the Google Art Project in the Extend Search mechanism in the section "Art Images". Here is a search for Vermeer. For more information on AUR Library's Extend Search, see the Two-Minute Tutorial.