I've posted thousands of pictures here, with a note that they can be used for non-commercial purposes only. Permission required otherwise. Twice, I've heard from people wanting to use the pictures. I hadn't really considered what to ask, but didn't want to get involved with anything complicated about declaring income, tax liability, charging sales tax &c.
The first time, I reflexively had a good instinct. I asked for a donation to a charity of my choice. I asked for $10 and they gave $20. The second time, just this past week, it was use of a family group photo from the 1800s for a Historical Marker. That time, I asked for $20 and they gave $25. I think I'll stick to $20 going forward.
I hadn't thought about it ahead of time. It felt like the right thing to do, and I think the people - who I thanked profusely for respecting intellectual property/copyright - were pleased too. I figured it might be worth mentioning here, as a suggestion to anyone else faced with a similar situation.