According to this document
All works published in the U.S. before 1923
is default public domain.
Does this mean it is safe to use images of historic figures (Isaac Newton or Leonardo Da Vinci) in our artwork? Is there exceptions to this ?
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Edit: this question has been edited after it was posted, but originally suggested that "historical people" are themselves copyrightable.
You should ask your lawyer for legal advice.
People are generally not copyrightable, although characters in a work of literature or art might be. It's work done by people which is copyrightable. Things such as works of art, sculpture, literature, movies, photographs.
I could take a photo of you today, in a public place, and I will own the copyright, no matter whether you agree to it or not. If this were not the case, the paparazzi would not be able to publish pictures of celebrities against their wishes. And fine art photographers wouldn't be able to sell works of people captured without their knowledge, or even display them in an art gallery.
I also would like to remind you that Leonardo da Vinci did not publish anything in the USA, since the country did not exist at that time. And Isaac Newton published his works in the UK, also at a time when there was no such country as the USA.
Edit: another thing that might be worth thinking about, just because Leonardo or Isaac are long dead doesn't mean images of them are out of copyright. An artist could easily have made a drawing in recent times, and that image will still be in copyright. The copyright belongs to the artist, not the person in the image.
Just because a person was alive a long time ago, does not mean that the images of that person, or images of their artwork is not under a copyright. The best thing to do is to do a search for images licensed under Creative Commons with usage that matches what you need to do with the image (i.e. commercial or non-commercial).
Two sites that I use frequently to search for CC images: https://search.creativecommons.org/ and https://visualhunt.com/, but most image sites out there have the option to search for images with a specific license--you want to look for images that are marked either "Public Domain" or "Creative Commons." And make sure to review the specifics about crediting the original creator for every image that you want to use.
Just don't make the mistake of thinking you can use "Royalty Free" images--the creators of these images will allow you to purchase their image for certain specific usages, but they are not the same as free images.
I would just use images that I am sure about. The article you mentioned has this section called "Where can I find public domain works?" with a list of archives where "the copyright term has expired or the creator has not renewed the license". Try to find your images here?
If you are planning to use these commercially however (you did not mention this), you should be very careful and try to get some written approval first.
Also see this question.