National Portrait Gallery (London, England)
Approximately 87,000 images from The National Portrait Gallery Collection are accessible for academic and non-commercial purposes. The licensing will vary by image. Click on the image, then underneath the image there will be Use this Image. If you click on that link there will be more information on the licensing of that image. The academic, "low resolution" image should suffice for many purposes at 2400 pixels per side and 300dpi. If you would prefer a high resolution image, please contact me and I'll acquire one for the VRC.