For hanging artwork in walls, the guideline is that the centre of the picture should be about 60" - 65" from the floor.  When hanging artwork above a sofa, place the artwork about 6" to 8" from the top of the sofa.  If you hang the artwork too high, it will look like it'f flowing away from the sofa instead of being connected to the sofa.  The artwork should not be wider than the sofa, it should be at least 2/3 of the length of the sofa.

The above also applies when hanging artwork above the headboard of a bed, a fireplace mantle, and a towel bar.  When placing artwork above toilet bowl, choose pieces are wide as the bowl to balance the two. 

If you are hanging more than one piece of artwork, space the frames 3" to 4" apart.