When a left hand has been stenciled, this implies that the artist was right-handed, and vice versa. Even though the paint was often sprayed on by mouth, one can assume that the dominant hand assisted in the operation. One also has to make the assumption that hands were stenciled palm downward—a left hand stenciled palm upward might of course look as if it were a right hand.
Could you please explain the grammar of the bolded sentence? I know 'hands' is the subject and 'were stenciled' is the verb. What does do the rest ('palm downward') of the bolded sentence do?
Also, is this a complex sentence or compound sentence?