Getty Open Content in the Getty Art Collection. Google Search results for site:www.getty.edu/art/collection/ “without charge” suggests there are currently around 1,700 such images in the Getty Open Content public domain programme. Sharp 20mb+ hi-res versions are available, though a simple anonymous form on which the user tells Getty the intended use.
The Getty has an Advanced Search panel which can limit the search results to Getty Open Content. A test for “rabbit” unearthed nine items, but failed to unearth a delightful rabbit in a 13th century illuminated manuscript.
The manuscript rabbit was however found via a straightforward keyword search in DuckDuckGo’s Image Search…
site:www.getty.edu/art/collection “without charge” rabbit
However I should note that DuckDuckGo Image Search also failed to find the other nine “rabbit” results.
Further perusal of the 17Mb source picture for the single-rabbit image found by DuckDuckGo, revealed there were actually two such rabbits on the manuscript page…
Thus a combination of a Getty Advanced Search and DuckDuckGo Image Search seems useful for public domain picture researchers.