Too bad you mentioned an artist who died in 1976 and you know better than me that there were no copyright rules, because the internet was far away.
Citing the sources, for the correctness of any work with images is an ethical duty.
For the rest, @jagur.force answer is exhaustive.
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" that there were no copyright rules, because the internet was far away."
you should only comment about a subject you have at least a basic understanding of.
I researched and compiled extensive resources concerning copyright.
What I am talking about is that sources get nitpicked, while there are all kinds of copyright violations going on, even by the author of this post.