Judging solely by the placement of windows and doors, it seems like it would have to be the rear room adjoining the kitchen and dining room.
However, I'm beginning to wonder if this really is the Ipatiev house. The room seems disproportionately large, with a very high ceiling. The chandelier is much larger and more elaborate than the other light fixtures in the house. The trim around the ceiling is also different -- all the other rooms I've seen have deep wood mouldings, but this room appears to have plasterwork instead. Also, all the other doorways I have photos of are arched, while this room's doorway is square.