I've lost count of how many times this photograph has been stolen from me. It shows Dunluce Castle, Co. Antrim on a fine day and is actually a photograph that I took on a scouting trip. I had intended to return for a photograph at sunset or sunrise, but I never made it back.
What makes this picture so worth stealing (among a certain group of people online, anyway) is that it shows perfectly how to use a graduated neutral density (ND) filter to balance bright and dark areas in a scene. These filters allow you to expose for the darker area of a scene, while helping you to retain detail in the lighter area in your photograph, which would otherwise just burn out.
I used this photograph to accompany an article about using graduated ND filters on the popular Luminous Landscape website. Since then, an endless stream of people have nicked it as an illustration of the use of graduated ND filters on their own websites and platforms.
(You can buy a print of this photograph here: https://buff.ly/2EgwwaK)