If you look closer, most of these aren't as random as you might think. House to Home uses a horizontal, straight line as a guide. There are pictures both above and below the line. The Domino gallery is centered above the couch, and most other galleries, though they're not perfectly straight, are usually rectangular shaped. Start in the center, then work your way out. Add other items like mirrors and typography. Make it personal!
image from House to Home
image from Mark Lund Photography
image from Domino
image from Suburban Renewal