Although it is not a rule of a thumb, we do recommend selecting night and evening photographs for your bedroom, or other places where you spend late hours.
Abstract photos and details can be mixed in great numbers. Keep in mind to preserve some balance of colors between photographs.
Decorative photographs can be used to add some color and interest to any empty spaces such as the kitchen or hallway.
Landscapes and seascapes can be treated as windows so big sizes are recommended for them.
Before hanging, arrange all photos flat on the ground to judge the effect.
Group pictures by color or subject. you can also change the color or the subject by introducing another photo in the middle that connects two different photos. For example, you could hang the sea-scape and the photo of a white village by introducing a photo of a house on the beach.
Treat the wall as a unique composition. If you have two photos of a coast, it would be a better idea to put them face to face, instead of back to back. The viewer's eye will stay longer contemplating the views. The symmetry of a set gives an elegant and clean look to the wall.
Consider both: vertical and horizontal symmetry.
Fix a small piece of double-coated tape in the bottom of the frame, so it does not tilt!
In a row: By placing an odd number of pictures in a horizontal line to each other give a sense of space. This works well in hallways or stairwells.
You can also arrange multiple pictures in geometric shapes like a square, rectangular and even oval. Make sure that inside those shapes there is some vertical or horizontal alignment for the pictures.