See the signature?
The photo was taken on the top floor of the Glasgow School of Art, a famous building from 1899 designed by Charles Rennie Mackintosh.
A promo photo from the No Little Boy period.
This was also used for the cover of the songbook John Martyn For Guitar Tab.
This picture was shot for Permanent Records. Probably around the time they released Couldn't Love You More (September 1992). It was not used until April 1997 though, for the cover of The Very Best Of.
This scan was taken from the Cooltide program. It must be a promotional picture in use by Permanent Records but they never credited it to my knowledge. My guess is it was taken by Julian Broad.