You should attribute your images. You are making money off of other people's art without giving them credit or compensation. Public domain images are one thing but, It's really not that hard to research and cite where your artwork is coming from (in one case here https://www.juxtapoz.com/news/street-art/edge-of-time-drawing-by-interesni-kazki/ ) and it's the "woke" thing to do.