along with size I'd consider how accessible the artworks are, as you point out volunteers can produce higher resolution but can they access the artworks tot take the images. Its small they wouldnt be usable for any other purpose, personally I'd be looking for a minimum resolution of 1200 on the longest edge which then enables people to look at the image in a size that they can see details, ideally being above 2400 then give people the opportunity to look at the smaller details of brush work and techniques which is critical encyclopedic aspect for artworks