I've been toying with the idea of selling paintings. I don't know if I'm good
enough yet that the income would outweigh the stress. But maybe one day...sigh.
When I do, Etsy might be the way to go because my shop is already well established there.
And then I need to decide if I'm selling artist prints or originals. Originals mean higher shipping costs and insurance. Very expensive in Canada. Print on Demand options mean less stress on my part, less income, but I can sell the same thing over and over again. It also means I need to confirm that I made it from my idea... aka, copyright boring stuff.
Basically, I probably couldn't sell my Wizzard paintings without a license from whoever holds the Terry Pratchett estate, but I could sell paintings of my own
chickens from photographs I took. I wonder if that's what the site you were looking at was meaning. Proof that you hold the copyright to all the elements in the picture.
Another idea I'm looking into is the digital market here on permies.
https://permies.com/wiki/64443/set-thread-Digital-Market I've sold some goodies on there before and it's lovely. Zero fee to set up a listing, very small fees when selling. The opportunity to set the
affiliate pay out to whatever percent you like (hint, 40% seems to be the sweet spot)