DIY: Gallery Wall Arrangement Ideas - plus many other guides, such as yardage guides for upholstered furniture, window treatment & terminology guides, area rug size guides, etc. This is an excellent resource!!!
Geometrical arrangement: Remember to give to the most important piece a central position, just at the eye level... Then, separate 2 frames with same size by a smaller one. This give to your lay-out rythm and energy.