As said, copyright law is complex, and that can mean expensive if you get on the wrong side of it....
In general, if you use someone else's creative talent in something you do, then the other person should be rewarded for their input.
That's what copyright aims to achieve.
When anyone creates an original work, that person automatically has copyright in that work (unless they have signed it away to someone else, such as a freelance worker; some employees or other circumstance).
In the UK, that copyright remains with the creator until 70 years after death. Other countries have different limits. See here:
https://en.wikipedia.org/wiki/List_of_countries%27_copyright_lengths
In general, therefore, if you use any image within that period, you *MUST* seek permission from the copyright owner. If you fail to do so, it is treated rather like theft.
Beyond that period, the question is moot. For example, if the National Gallery takes a photo of a picture by Leonardo da Vinci, the photograph is protected by copyright.
So even if you find an image of an artwork where the painter has been dead for a couple of hundred years, it may still be within copyright.
Much the best is to find an image where the auther has specifically said it may be used for free.
One way is to go to
https://search.creativecommons.org/
make sure to check the box for 'use for commercial purposes' and then do a search - it should identify images where the owner permits people to use them for commercial purposes. But always check first.
It would be kind (and may be part of the arrangement) to credit the copyright holder.
Good luck!