ClinicalKey is a great place to begin searching for images from textbooks and journal articles. In the main search bar, select "Multimedia" in the dropdown menu. Once you search, you can further limit your search with filters.
TCIA is a service which de-identifies and hosts a large archive of medical images of cancer accessible for public download. The data are organized as “Collections”, typically patients related by a common disease (e.g. lung cancer), image modality (MRI, CT, etc) or research focus. DICOM is the primary file format used by TCIA for image storage. Supporting data related to the images such as patient outcomes, treatment details, genomics, pathology, and expert analyses are also provided when available.