There is no single way to hang pictures in a stairwell. Instead, it is more of a personal choice. The most important thing to remember is to place the pictures near eye level to make it easier for people to see them as they walk up the stairs, especially if there is just a single row of pictures.
A simple picture placement is to hang the pictures in a diagonal line at the same angle as the stairs ascend or descend. If they are photos, one can switch between vertical and horizontal pictures to create a pattern.
Another option is to fill the wall with pictures of all sizes. Little pictures can be mixed in with larger pictures to create a wall of art. A single picture can also be used to make it stand out. Place it in the middle of the wall so that it creates a focal point.
To decorate the stairwell, one can mix the pictures with mirrors and other decorative elements. Using various colors and styles of frames creates a mix of light and dark. Another option is to choose one color of frame and stick with it for all of the pictures. This creates a more dramatic look.
There are no rules in putting pictures in a stairwell. The photos can be placed in organized lines or in a hodge-podge fashion for a more eclectic look. This area should reflect the personality of the person that lives there and should match other areas of the home or building.