In public Domain
** Images by the U.S government are automatically public domain. Probably those of state governments as well.
http://www.cdc.gov/media/subtopic/images.htm
http://phil.cdc.gov/phil/home.asp
commons.wikimedia.org (simply type and search the disease or condition. E.g. Tinea Corporis)
http://www.public-domain-image.com/