OK, that was rather silly of me. in THAT diagram, Black STILL has no valid previous move.
If the thing discovering the check is a knight instead of a bishop, it could be done. But White has both Knights. It cannot be a promoted Knight because there is nowhere for White to promote - White's missing pawns have to be the G and H pawns, which do not have any path to promotion.
EDIT: Wait, I think I found it:
The key, of course, is, if it's White to move, what was Black's last move?
Everything on the 8th rank is trapped. (And that actually IS the 8th rank, that's listed, so the board isn't flipped or anything.)
fxe6 is impossible since the h5 pawn has no way to get there unless it's the F pawn. Similarily, h6-h5 is impossible.
So that leaves king moves. Since the king is trapped, the last move by White must have been getting Black into check. But the White rook is very limited by the king and rook next to it. O-O is impossible, as someone stated, because it implies the White king never moved, and then Black has no way to get its king to that part of the board.
Which leaves Kxa2. I frankly see no reason why the following is impossible: