Free being 'gratis' in this case. There are additional licensing terms that make it non-libre/non-open/non-free (as in freedom) [0].

Finding quality libre or public domain art remains difficult. Here are a few resources that I've found:

Archive.org - https://archive.org/

FOSsil Bank Art(Libre) - http://fossilbank.wikidot.com/category:art-libre

Flickr British Library - https://www.flickr.com/photos/britishlibrary/albums

Artnome - https://www.artnome.com/art-data

Comic Book Plus - https://comicbookplus.com/ (public domain comics)

And of course, Wikipedia has libre-licensed art. I'm a little embarrassed but PublicDomainPictures and PublicDomainVectors is also a go-to resource for me.

[0] https://pixabay.com/service/terms/#license