(U.S. Copyright Office Definitions; emphasis added)
"A work enters the public domain on the first of the year following the expiration of its copyright term" (Cornell), which is now 95 years in length. As copyright law has been amended several times in order to extend copyright protection, creators who so desire can opt to put their work into the PD immediately, often through the use of Creative Commons licenses.
Works that are in the public domain are free to reuse and repurpose. 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.
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 a paper, you must include a full citation of the image in your list of works cited:
The information presented here is for informational purposes only and should not be considered legal advice.