However, she is tapping her foot... The yellow Maria on the right isn't animated at all - but if you made her animate, I fear that would also cause the Foot guardian to start flicking to the Barrel sprite and back again? (Since it shares the same Guardian Class).
I've just realised that it would be possible to make the yellow Maria in The Nightmare Room animate through two, or indeed all four of her sprite-frames, if you wanted her to, without causing the yellow foot guardian (which shares the same guardian class number) to deviate from the foot sprite - but only if you swapped around the position of the foot and the barrel on their sprite page (i.e. Page #9C - put the barrel at #9C40 and the foot at #9C60).
Of course, you would then have to adjust other instances of a foot or a barrel guardian elsewhere in the game, and also amend the pertinent addresses in the 'Game Over' routine accordingly.