I can see 4 images on the walls. But 3 pstones to spin... I have no idea what to do.

You'll notice that there are three rotating stones, and two doors. First, notice that the two outside stones have small animal symbols just next to them on the wall. Rotate those outer stones so that the symbol on each stone is facing its corresponding counterpart on the wall (e.g. if there's a snake on the right wall, rotate the right stone until there's a snake facing the snake on the wall. Now, the middle stone determines which door opens. Notice that above each door is a symbol. If you rotate the middle stone such that the symbol facing (toward the level and doors) matches one of the symbols above a door, that door will open. So you can actually open both doors to get the loot on the right, and then to move forward on the left, by rotating just the middle stone.