The Art Institute of Chicago now has 44,000 items from its collection downloadable as pictures under a CC0 licence. I did a test search for cat. What struck me first was the rich range.
My excitement was dampened when I realised that most of these results had no hi-res download. What I should have done was spotted the easy-to-miss faded “filters” button, up top, which when clicked pops out a sidebar. In the sidebar you can tick to filter by “Public domain”, which gives you the results with the downloadable images.
The filtered results are still fairly impressive, but of course lack the nicer “wow” illustrations made after about the 1910s. Some images download without file extensions, possibly because they already have a . in their title (e.g. “Honorable Mr. Cat”)…
The pictures seem to mostly be around 2,000 to 3,000px and 96dpi. There’s no sign-up needed, and access is free and public.