Legal to make copies and re-sell illustrations or photographs that are in the public domain?

I have some illustrations that are in the public domain that I would like to re-sell copies of them. Do I need to alter the image in some way before I can resell it?

I have other illustrations that are old, but it is unclear if they are in the public domain. I am confused how these laws work in general. I see people online selling images of things like the Batman symbol or Disney characters, putting these images on all sorts of things, shirts, mugs, etc. Are all these people breaking the law? It even happens on websites like Etsy where they take copyright more seriously, or so they say. Some people have had these Etsy shops open for years and tens of thousands of sales. Could someone come after them and sue them?

Is there a difference in the copyright law between photographs and illustrations?

Is there any way to find out if the copyright has been renewed?

Thanks in advance