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There is a lot that goes into making a good Facebook cover photo. First of all, the available space is 851x315 pixels. That is also a 2.7 aspect ratio, which is a really wide photo. The cover photo is intended to be a header for your page. The space is not suited for a narrow, horizontal slice across any photo you happen to like. So, you need to start with a photo that is composed to fit the space. This photo works because the pier and the shore create a natural horizontal line across the image.