Thank you, I found this link for the free content license: https://pixabay.com/service/license-summary/
and I am sure It’s ok to submit it to the marketplace since I modified it to make the background transparent, and it is not standalone, and it also doesn’t break Epic’s rule about public domain content: “Unmodified public domain content is limited to assisting with presentation, and sources are cited in the description”
I sent this message in case anyone else has a problem like this.