See the signature?
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 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.
One of the most famous pictures Anton Corbijn took from John. The location must be the environments of his Roberton cottage. As to the date, it probably was taken late 1984 as the back picture of Sapphire (released November 1984) seems to come from the same session. Also it was used by NME in Richard Cook's review of Sapphire (where John provided the names of the sheep by the way).