The French win. Of the three websites on my bookmarks toolbar for researching drawings, two are French: Joconde and the Louvre. The other is the British Museum. I return to these again and again. You’d think that America, the land of computing and the World Wide Web, would have magnificent, complete resources, but no. German electronic resources are also uneven.
The legacy of Diderot and the state structure of French museums have made their research websites remarkably strong.
The Louvre has 140,000 drawings online. For results, each page delivers 5 entries, mostly with thumbnail images that are sufficiently readable (unlike the BM, where the thumbnails are a bit small). If there are too many results, say for Stefano della Bella (there are 688 results or 138 pages) and it becomes laborious going through them all, I then switch to Joconde which gives 100 results per page, and includes the Louvre and other state collections. The Louvre’s images are richer, so I weave back, via inventory number (listed under oeuvres), to view the better images.
The British Museum’s database is also close to heavenly. Results include prints which can be good, but also overwhelming.