Public domain image resources
Public domain image resources is a copy of the master Wikipedia page at Meta, which lists a number of sources of public domain images on the Web. Public Domain images should be marked with the Public Domain Mark 1.0. Public Domain Mark enables works that are no longer restricted by copyright to be marked as such in a standard and simple way, making them easily discoverable and available to others. The Public Domain Mark is recommended for works that are free of known copyright around the world. These will typically be very old works. For a creator to release his/her works into the public domain legally they must use the creative commons CC0 license which gives creators a way to waive all their copyright and related rights in their works to the fullest extent allowed by law. The presence of a resource on this list does not guarantee that all or any of the images in it are in the public domain. Please read the policy on image use and etiquette at: Wikipedia:Image use policy.
Related: citing referencing