Peter Freestone states that when the book came out, the "1981" caption was a typo, it should have read Autumn 1991 instead. So probably October/November 1991. He also says in the book that after Freddie returned from Switzerland on November 9th, he spent most of his time in bed..although it could have been from (just) before he went on that trip. I wouldn't think it would be any earlier than September 1991. As I say, most likely post-Montreaux.
If i remember rightly freddie had a very quiet birthday in 1991,
and his mum & dad were told not to attend as freddie did not want them
to see how ill he was.
So.... the picture must have been ocobert 1991.