These images were distributed under the CC0 "no copyright reserved" license when I downloaded them in 2018 from pixabay.com . They changed to a more restrictive custom license[1] in January 2019[2]

Image URLs:
https://pixabay.com/photos/waterfall-valley-mountains-1081997/
https://pixabay.com/photos/bastei-bridge-saxon-switzerland-3014467/
https://pixabay.com/photos/rustic-cabin-mountains-landscape-945421/

1: https://pixabay.com/service/license/
2: https://en.wikipedia.org/wiki/Pixabay#History

Ok, please edit the license file to reference where we got them (something we do consistently for ethical and auditing reasons). It doesn't have to be very specific.
And then you can mark this resolved.