Rolling Stones' Brian Jones and the mountain dulcimer on stage
Just two pictures found on the internet when doing some research. The two songs Jones played on the dulcimer were 'Lady Jane' and 'I Am Waiting' from the album Aftermath. The album was made in December 1965, so the performances using the dulcimer will date from 1966.
The instrument was electric amplified, probably by a guitar like pick-up. It seems to be a rather large instrument, although I can't find any measurements about Mr. Jones' proportions...