Reminder: Using images in the public domain does not require copyright permission or a fee, but you should always cite the source of the image and its owner.
Centers for Disease Control and Prevention Public Health Image Library
http://phil.cdc.gov/phil/home.asp
Centers for Disease Control: Vaccine Preventable Diseases
http://www.cdc.gov/vaccines/vpd-vac/photo-all-vpd.htm
MedlinePlus
http://www.nlm.nih.gov/medlineplus/
National Cancer Institute
http://visualsonline.cancer.gov/
http://nccam.nih.gov/news/multimedia/gallery
National Eye Institute Photos, Images and Videos
National Human Genome Research Digital Media Database
https://www.flickr.com/photos/niams_nih/galleries
National Institute of Biomedical Imaging and Bioengineering Image and Data Repository
http://www.nibib.nih.gov/Research/Resources/ImageClinData
National
http://catalog.niddk.nih.gov/imagelibrary/
National Institute of General Medical Sciences Image Gallery
National Institute of Mental Health Image Library
http://infocenter.nimh.nih.gov/il/public_il/
National Institutes of Health Photo Galleries
https://www.nih.gov/news-events/digital-media-kits
National Library of Medicine: Images from the History of Medicine
http://www.nlm.nih.gov/hmd/ihm/
National Library of Medicine: Medical Image Database from the Armed Forces Institute of Pathology Electronic Fascicles