Best Practices for Accessible Images" at California State University, Northridge Universal Design Center. This also includes a helpful section on image captions.
Following are a few free or low cost sources of images that are either public domain, or that you have the right to use by becoming a member of a service or by subscribing to a service.
Please read each site’s Terms and Conditions regarding use and image credits.
Image Quest (available for PCC faculty, staff and students for PCC-only non-commercial educational use)
New York Public Library Digital Collections: Public Domain Collections