The guidelines offered here should be taken as suggestions, not as legal statements.
The re-use of images for educational purposes (not including print or electronic publication of any kind) is generally considered acceptable under the terms of fair use. If you wish to publish images online or in print, even if for educational purposes, you will first need to determine whether or not the image is protected by copyright, then find out how to get copyright clearance.
Password-controlled web sites with access limited to the Swarthmore College community generally fall within the bounds of educational fair use.
When in doubt, consult the Cornell University Fair Use Checklist or the Fair Use Definition (U.S. Code TITLE 17, Chapter 1, Sec. 107).
You may also need to obtain permission to publish from the institution that owns the image in question, whether or not the image is in the public domain. This is particularly the case for images found in licensed databases, such as ARTstor. ARTstor has a very clearly-worded permissions statement, as do other licensed databases. In most cases, you will need to write to the institution that owns the physical image (that ARTstor, for instance, includes) and request permission to publish it. There is often a fee associated with acquiring permission to publish.
Example of a museum statement on rights, terms and permissions of image use for works in its collections:
Example of an image licensing company:
Art Resource (manages licensing for major art museums and archival collections)
Creative Commons is a non-profit organizations that allows creators to retain copyright while letting users share and distribute their work under certain restrictions. As you search images and find works under a Creative Commons (CC) License, please pay close attention to the type of CC license the creator holds. As per the Creative Commons site, if you are not sure what type of CC license the work holds, contact the creator to ask for their permission to use the image. Below are the 6 main types of licensing when using CC: