As some of you will know, I maintain a Free Space Page for galleries and collections which explicitly allow use of images on WikiTree. In some cases, I have secured permission by email.
There is a volunteer-run database for US "historical information viewed through the filter of roadside and other permanent outdoor markers, monuments, and plaques": https://www.hmdb.org/. This contains lots of images, with copyright generally shared between the site and the photographer. It allows copying onto WikiTree provided minimum accreditation is given.
I have added this site to the Free Space Page, with guidance on accreditation wording.
Please note that a minority of images should not be reproduced without the specific consent of the rights holder. Where this is so, it is clearly indicated.
Finally, if you know of galleries and collections which explicitly allow the copying of images and which are not yet on the Free Space Page, please contact me.
[corrected for minor slips and typos]