To find images available in the public domain for use in projects, you can use the resources linked below to start.
Wikimedia: check item record for permissions.
Creative Commons: check the CC license for limitations on use.
Google Images: use the tools menu to limit your search by the "usage rights" of the image.
Public Domain Pictures: a repository for free public domain images.
The Public Domain Review: a curated collection of openly licensed images from libraries and museums such as the British Library, the Rijksmuseum and the UK National Archives.
Pixabay: high resolution photos, vectors, and art illustrations for free use.
Intellectual Property and the Arts: the College Art Association's guide on fair use, permissions, and copyright in the visual arts.
You can also use the "How to Give Attribution" page on the Creative Commons website as a guide for how to cite these images.
To find music available in the public domain for use in classroom projects, you can use the resources linked below
YouTube Audio Library: designed for creators looking to add copyright free music to their work.
Musopen: this source archives open access recordings of classical music
PD Info: Searchable list of public domain music
Medical image resources will provide varied options for locating copyright information and use guidelines may differ even in the same database.
The default display for PMC search results is the Summary view:
The following article includes a variety of clinical images.
Copyright and License Information is found below the author names by selecting the title link to display the full text.
Images from the History of Medicine (IHM) in NLM Digital Collections provides online access to images from the historical collections of the U.S. National Library of Medicine. IHM includes image files of a wide variety of visual media including fine art, photographs, engravings, and posters that illustrate the social and historical aspects of medicine dating from the 15th to 21st century.
Historical Collections Copyright Information includes specific guidance, including Prints and Photographs, Films and Videos, Online Exhibitions.
MedPix® is a free open-access online database of medical images, teaching cases, and clinical topics, integrating images and textual metadata. Images from the MedPix database are accompanied by 
which will link to the text:
"MedPix® images and case materials were contributed by many individuals. They are organized, reviewed, approved, and curated free of charge for your personal use and for local teaching at your institution - including distribution of handouts and syllabi. For anything other than personal use, you should respect the original contributor and contact them for additional permission requests."
The National Cancer Institute (NCI) Visuals Online provides a range of photographs and illustrations that may help communicators of medical and public health convey meaning, describe concepts, and tell stories. The collection spans everything from biomedical, science, and patient-care related illustrations and images to portraits of present and past directors and the staff of the National Cancer Institute.
Each image or illustration included in the NCI Visuals Online Web site is labeled either "Public Domain" (free to use) or "Copyright Protected" (restricted, obtain permission before use). Additional detail included for acknowledging public domain images and steps to take for copyrighted protected images.
This is a searchable collection of scientific photos, illustrations, and videos. The images and videos in this gallery are licensed under Creative Commons Attribution Non-Commercial ShareAlike 3.0. This license lets you remix, tweak, and build upon this work non-commercially, as long as you credit and license your new creations under identical term
The NLM Visible Human Project has created publicly-available complete, anatomically detailed, three-dimensional representations of a human male body and a human female body. Specifically, the VHP provides a public-domain library of cross-sectional cryosection, CT, and MRI images obtained from one male cadaver and one female cadaver. The Visible Man data set was publicly released in 1994 and the Visible Woman in 1995.
"Open-i is the service of the National Library of Medicine that enables search and retrieval of abstracts and images (including charts, graphs, clinical images, etc.) from the open source literature, and biomedical image collections."
May I reuse an image I found through Open-i? "We cannot grant permissions to use images. Copyright remains with the authors or the journal. A link to the copyright license type may be found below the individual Open-i image on the detailed view page. If copyright restrictions for an image are not specified or are unclear, please contact the journal that published the image."
Photographs, illustrations, and multimedia files, from the Centers for Disease Control and Prevention, organized into hierarchical categories of people, places, and science. The Details page for each image provides the "Copyright Restrictions." For example, the dysplastic nevus record indicates: "None - This image is in the public domain and thus free of any copyright restrictions."
The Thomas Jefferson University - Center City Archives & Special Collections points to where the university is heading by documenting where it has been by acquiring, preserving, and making available for research official university records, personal papers, memorabilia, and other materials of enduring historical value that document the development of Jefferson and its constituent institutions, affiliates, and alumni.