The websites Endoszkop.com, AcupunctureProducts.com and ClevelandClinic.org provide free printable images of the shoulder muscles and bones. An article about the human shoulder on Wikipedia.org also features images of the shoulder that are freely downloadable.
Endoszkop.com features printable pictures of the human anatomy, including the shoulder. The images can be downloaded in different resolutions.
AcupunctureProducts.com provides free printable diagrams of the muscles and ligaments of the shoulder. Users can download the images in PDF or in JPG format.
The diagrams at ClevelandClinic.org are simpler, black and white images of the shoulder.
The Wikipedia page about the shoulder contains free diagrams of this part of the body. The images are downloadable and printable.