(Photo Credit- discover: interior design)
"Continuity is essential in a grouping. Many say that the outside perimeter of the pictures should follow the lines of the area being filled; If hung over a piece of furniture or cabinetry, the lines should also relate to the unit below. I think that’s perfectly rational thinking, but that doesn’t mean your outside perimeter must line up perfectly. In the example below, we compare a blank arrangement that possesses continuity and one that doesn’t."
"Balance within a grouping is important. Visually heavier pictures or ornate frames work well when interspersed with lighter pictures using simpler frames. Without this precaution, the composition will seem divided, with no unity. Unity will be maintained if larger pictures are placed at either side of the grouping with smaller ones within."
(The tips above are quotes from Designer Kristin Warbington of http://discoverinteriordesign.com)