There are pictures all over the place, but the only special one is as far away from the enterance as possible.
Head to the opposite side of the castle, to the part where you are over a balcony, the thone room is below you.
Head thru the blue portal at the top of this room, that will teleport you to the room below (throne room).
The special picture is in that room (north side wall, slightly left of center).
I can't remember if the special picture turns into a portal to a special room, or teleports you to special room.
Anyway, in the special room is the toughest undead army in the castle. Can't really remember the treasure, but I think you got an option to give (some/all?) of it to King Mark.
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ufc