A few weeks ago, I received the following email:
I moved from Scottsdale, Arizona, to California last summer, and brought an unfinished painting of Papago Buttes along with me. I looked around for a photograph to help me finish the painting, and I found one that seems to be attributed to you on Wikipedia. The shot helped me enormously, and I ended up finishing the painting and giving it to a friend.
The more I learn about artwork and photography, the more I realize that asking permission before using a photo, even when referring to it for painting, is the right thing to do. I just wanted to be in touch to apologize for failing to do this, and to offer to email you a photograph of my finished painting. I think you’re a good photographer and you helped me by sharing your image online.