Featured Collection: The British Museum
The British Museum has over 500,000 of their collection images available through their online database. The images are available in download sizes of 1024 pixels wide, or greater, and are approved for non-commercial education use.
In addition, the British Museum offers high-resolution images (2500 pixels wide) free of charge, that can be used for non-commercial publications with a print-run of less than 4,000. You may order up to 100 of these high-resolution images per month. See their website for more information on obtaining their high-res images.