I took some images that are free to reproduce and resized/cropped them to the size of the current banner (1170x240). Unsplash suggests giving credit to the photographer, but it's not a requirement.
Credit: Word of the Lord by https://unsplash.com/@tannermardis
This next one is my favorite, but I promise I won't get my hopes up:
Credit: Tor's Fight with the Giants by Mårten Eskil Winge (1825-1896), photograph from Wikimedia Commons.