Do exactly like it says. There is a 3-way parity constraint; the three pieces dropped between two pictures add 12 blocks, and each line dropped removes 9, so each picture has 3 (mod 9) more blocks than the previous one.

One of these transitions didn't seem to work, but the Tetris pieces needed to make things work and the order of the photos is shown below, with the three unused photos at the bottom.

The letter shifts work as described in the puzzle, and as indicated in the solution above, spelling VA STATE IS ANSWER, so the answer is VA STATE.