The Wellcome Library has recently made their large database of images on the history of medicine and science <http://wellcomeimages.org/> free for both personal and commercial use, meaning that authors can incorporate them into book projects free of charge and hassle. This is one of many collections that have decided to release their images under a Creative Commons License < http://creativecommons.org/licenses/by/2.0/uk/> and that provide high-resolution, reproduction-quality images.
The same goes for Manar al-Athar < http://www.manar-al-athar.ox.ac.uk/>, a resource for images relating to Middle Eastern studies.
Many art historians are not yet aware that Artstor, one of the largest databases available of canonical works, has a special program called Images for Academic Publishing < http://www.artstor.org/what-is-artstor/w-html/services-publishing.shtml> whereby authors can easily obtain the images they need. Not all images in the database are part of the program, but quite a few are (each of these has the IAP icon next to it). These are available free of charge for print and e-books with projected sales of up to 2,000 copies. This includes images from the Metropolitan Museum of Art, the Getty Collection, and the Indianapolis Museum of Art, among others.
To be continued!