Go to the home app and open the TV 'device' while the image is up. You should see the picture and a link to the artist's profile (or other relevant info).
You might be lucky enough to find it on the wallpaper stream page. https://clients3.google.com/cast/chromecast/home
Have you tried using Google lens?
Google the creator, and you'll find the image and its location. Also, if you have the Google home app, you can look at different slideshows and view the creators info.