I've had that happen a few times. I just say no and move along. It's weird to ask a stranger to take a photo of a tattoo on their person.
Just this week, a little old lady touched my fresh elbow tattoo and told me how pretty it was. I said thank you, but I told her it's quite new so maybe don't touch? She covered her face in embarrassment and apologized profusely. She was sweet and didn't know better.