The Wheel of Cyclic Existence
The Buddha and the moon: liberation
The moon on one side indicates liberation. The Buddha is pointing to the moon, indicating that the liberation that causes one to cross the ocean of suffering of cyclic existence should be actualized.
The fierce being holding the wheel: impermanence
The fierce being holding the wheel symbolizes impermanence; this is why the being is a wrathful monster. I once had such a painting drawn with a skeleton rather than a monster, in order more clearly to symbolize impermanence.