A LinkedIn company cover is a shallow, wide banner. It is different from a personal background and different again from the images used in Career Page modules. Start by checking the placement you are editing. A file that worked for another part of LinkedIn may have the wrong proportions even when it looks like a banner.
LinkedIn's current Page image specifications list 1512×256 for the Page cover. This page uses an exact custom export at that size. Older templates may use other dimensions, so inspect the original design rather than stretching an old banner to fill the new rectangle.
Choose a photograph or a restrained composition that makes sense in a narrow band. A large group portrait is difficult to crop without losing faces, and a poster full of details may become unreadable. The company logo has a separate place in the Page layout, so the cover can communicate setting, work, or identity without repeating a tiny version of every brand element.