Welcometo Gamedev and hopefully we can help you solve your many many questions. One piece of advice - Don't take what Tom Sloper is saying to you to heart. Each forum is specialised to a degree and what you will find is that you may not obtain all the answers you need in one forum. Specifically legal/business questions are more likely to find an answer in this forum, whereas questions about Art tend to belong Visual Arts etc. You may note the top of each forum list is divided into Technical, Creative and Business. When Tom is moving you around what he is endeavouring to do is locate you in the best forum for which you could obtain most of your answers (not an easy task given the range of them tbh). Where he advises the idea of splitting up questions so that they can be matched with appropriate forums, he is giving that advice in order to enable you the best access to the right people. You will find that most people in gamedev tend to inhabit their own corners of the forums and wander rarely though there are some who do so.
First of all -- Whilst there are general rules of thumb for copyright -- there are also many differences in copyright between differing countries. The first piece of advice is - consult a lawyer appropriately qualified to provide you insight into your country's specific differences. If you are unable to afford a lawyer, I might suggest checking with Art councils, legal aid etc for people who can give you insight -- these usually tend to be free with their advice and somewhat knowledgable of local conditions.
Second of all - because copyright is such a pain in the proverbial "donkey" -- it is often much simpler to say something more specific for example "I want to use the Mona Lisa picture as a visual - Can I do this?" Specific questions like that are a lot easier to obtain answers for (most of the time).
Okay Artworks (I shall refer to all visual mediums by this term rather than individualising except where appropriate) that have entered the Public domain are no longer copyrighted, BUT and this is where the fun starts. If for example you wanted to use the visual of an artwork owned by a museum you might run into problems. Many museums and similar bodies assert an argued right that they have rights to images taken of their "owned" public domain artworks. This has never really been established conclusively to anyone's happiness but one general rule of thumb does extend from this, a photograph taken of the artwork itself with no alterations made whatsoever appears to be sufficiently protected legally.
Normally a copyright on artwork will exist for the life of the artist responsible plus 70 years, therefore you may consider that any artwork where the artist died on/before (looks at date) 13th April 1942 as being in the Public Domain. I wish that was true . The exception to this is "estates" - said artist dies but the estate holder may be granted an extended copyright. For example:
The court ended up ruling, by a 7-to-2 vote in 2003 in Eldred v. Ashcroft, that extensions for works still under copyright are allowed.
This month, the court agreed to hear a case on the question Justices Breyer and Souter anticipated, one that will test whether there is indeed a constitutional line Congress may not cross when it comes to the public domain.
The new case asks whether Congress acted constitutionally in 1994 by restoring copyrights in foreign works that had belonged to the public, including films by Alfred Hitchcock and Federico Fellini, books by C. S. Lewis and Virginia Woolf, symphonies by Prokofiev and Stravinsky and paintings by Picasso, including “Guernica.”
Lions, tigers and bears! Oh, my! You might be saying at this point and you wouldn't be too far from the truth.
Works that have entered the public domain that have been absolved of their copyrights by the owner (and here it can get contentious the owner and artist who made it can be different people and has at times caused legal cases to crop up when said artist says you can't do that to my copyright!). The other issue that also becomes concerning is the growth of plagiarism on the internet. Many artworks have been attributed as having been created by an artist different to the actual artist. So there is the potential minefield of downloading a public domain graphic that turns out to have been a plagiarised copyrighted graphic. The truth be told though given the sheer plethora of free art on the net - you may never ever know who if anyone holds copyright or whether it is public domain.
So much for public domain works.
Okay onto Artworks still under copyright - you will need to obtain the permission of holder of the copyright in order to use it without legal sanctions.Simple as that. I am not going to go into issues of fair use etc and all that because tbh I don't think it extends into what you were asking about.
Some basic advice -- It is cheap, effective and utterly free to make your own artwork as placeholders. If you want feedback on how the artwork looks or how it could be improved -- then by all means throw it into the Visual Arts forum for advice and feedback. You might however give basic consideration to scouting a local talent in your area and bribe them with gifts and flowers etc to do a set of pictures for you that aren't difficult and take no time and convince them to let you have copyright.......Make a contract if you do that.
So any image that is in public domain you can use - WITH EXCEPTIONS!.
Any image still copyrighted you can also use - IF YOU HAVE GAINED PERMISSION FROM THE COPYRIGHT HOLDER!
I hope that this post has not depressed you and I truly do hope that it helps you understand somewhat about this morass of legal spaghetti.