While you are thinking of where to place your artwork, you have to take into consideration the overall size of the item. To give you an example, putting small painting on a large small will dwarf its size tenfold. Then again, placing multiple paintings together on one wall surface makes it look cluttered.
A Focal Point in the Artwork
A general rule of thumb is to make use of one large photo or painting in creating focal point for that given space.
It may be hung above big pieces of furniture. For smaller artworks, they could be grouped together to be able to come up with a set that may be used as replacement for one large piece of art.
Just at the Right Height
Paintings must be also displayed at height where they can be viewed comfortably and thus, has to be at the eye level. It must not cause strains to the viewers while looking at the artwork. You may align the center of painting at your eye level.