Hi.
I searched the Internet for [royalty free images].
The description of this site in a Google search result: Search through thousands of royalty free images on Pexels. You can use all images on Pexels for free, even for commercial use.
If you find sites with a description like that, you are safe.
https://www.pexels.com/royalty-free-images/
In the past I have found sites with lovely clipart that I wanted to use in materials I made for ESLP / materials I thought I may upload to ESLP as well as using them myself.
I wrote an email to the site admin and stated honestly that I was making material for an ESL-worksheet contribution-based site called eslprintables.com.
I stated that it was not commercial.
I asked for permission to use the clipart (that gave me a digital record that I had permission from the copyright owner).
I stated in the eslprintables worksheet descrition where the images were from and that I had written persmission to use them.
No images contained watermarks.
I had no problems doing that.
Perhaps you could try that?
At least now you have the site name above and this information to start with...