Using a large computer monitor select a diplay page below then actively resize your viewport.
Landscape 1080p - High Definition TV format.
Landscape 4x3 - Standard most used by digital cameras.
Portrait Image - Standard most used by digital cameras.
In my opinion Landscape 1080p is the best format for landscape images.
In my opinion Square is the best format to photograph for display on a website any object that is higher than it is wide.