"The public domain is not a place. A work of authorship is in the 'public domain' if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner."
(U.S. Copyright Office Definitions; emphasis added)
However, please note that material found on publicly accessible websites does not mean the material is in the public domain. Internet content from websites -- including social media platforms and audiovisual repositories such as YouTube and Instagram -- are protected by copyright and the websites' terms of use and cannot be used without attribution and sometimes explicit permission.
These images want to be used and you should most definitely do so, but please cite the source institution. Though many of the public domain images are long out of copyright, citing the source institution (that is, the museum/library/etc. wherein the original image/material is held) makes it easier for others to track down the originals of the images you use. If you use any of these images in an assignment, publication, or course presentation, you must include a full citation of the image in your list of works cited.
Portals to search through the holdings of a number of institutions.
These works enter the public domain once their term of copyright protection has expired; more recent works can also be designated for public use at their time of creation.
Detail from Militia Company of District II under the Command of Captain Frans Banninck Cocq, Known as the ‘Night Watch’, Rembrandt van Rijn, 1642
Source: Rijksmuseum (click for full image)
DISCLAIMER
The information presented here is for informational purposes only and should not be considered legal advice.
