The J. Paul Getty Trust launched its Open Content Program which saw the release of 4,600 high-resolution scans of works from the Getty Museum in Los Angeles as open content. This means that the digital images in this new release can be downloaded and re-used without restriction and without the need to get permission.
Project Gutenberg offers over 42,000 free ebooks in the public domain. Search for images from these texts at the link above.
MERLOT = Multimedia Educational Resource for Learning and Online Teaching; peer reviewed open and online resources for faculty, staff and students in higher education.
Specific to Health Sciences is the database of images from Patrick Lynch from Yale University which are licensed under a Creative Commons Attribution license.
If you are looking for images specific to Health Sciences the Gray’s Anatomy database of images in Wikimedia is a fantastic place to start. All of these images are now available in the public domain.