2 million pictures extracted from the Biodiversity Heritage Library: Search: bookcollectionbiodiversity | Flickr. Size and crispness is of course variable, due to the nature of the materials, but there are plenty of good 2MP-sized picture to be found.

Not currently well indexed by Google Images, so one can’t do a site: + “Larger than 4MP” search in a useful manner. Nor is Flickr’s own search very good, seemingly. For instance, from 2 million images Flickr can only find 7 for the keyword “flower”…