Search for artists alphabetically, and bring up a list of links to all the good-quality images of their works on the web. Mostly text, but can have quite good images. Each link is annotated to tell you which works are illustrated, and the scans are excellent. Mark Harden's Artchive - a good resource, if slow to load. It includes one of Leonardo's drawings, which is unusual.
Quality is superlative.
The first floor has Renaissance and Baroque painting and sculpture. Very nice site. This site is very slow because all the thumbnail images are grouped on a single page per period. But give it time. They are excellent images.