I managed to find the magazine, but couldn't find an article, then checked the letters page and voilà!
I don't think it'll help you locate it unfortunately. It's a rather lengthy letter, waffling on a bit, but the important bits are:
"....we landed ashore close to the River Coruisk, on the Island of Skye. Our destination was the loch of the same name....It was voices across the water that first caught my attention, and prompted me to train my binoculars on the other side. Two people were trying to cross a fast-flowing stream. As I focused on them, and the surrounding area, I saw clearly and without any doubt, the figure of a man cut into what I took to be rock; he was lying along the water's edge, the face of a young, lean angular man with what looked like a crown on his head. To the left of his head (from the top of the crown) was an arrow, broad and coming to a sharp pencil point, then a female face, this face was not so clear perhaps because I was not directly opposite her. How did I know it was a female? I'm not sure now; I think it was a softness in the face, there were strands of hair flying around that face. I was also looking from an upright position, on figures lying sideways.
On the way home, I tried to rationalise what I had seen. Someone had been chiselling, or painting the rock face. Lots of people must have seen it or known about it. I would soon find out what it was. Was it Christ they were depicting? No one I have talked to so far has seen it. Has anyone?"
There's an address given. Probably better not to write it here, but I can pass it on via e-mail if you like (my address is andy at stravaiging dot com). It was 12 years ago, but she does seem to still be active in the area (eg this church newsletter