An X header is much wider than a typical photograph. Cropping a standard landscape image to that shape can remove sky, foreground, or the top of a person's head. Choose a source with enough open space around the subject so the crop can become a narrow band without looking accidental. A portrait is rarely a good starting point for a header that also needs readable wording.
X recommends 1500×500 header images. The recommendation describes the upload shape, not a promise that every pixel remains visible in every profile view. Screen sizes, browser layouts, and the profile picture can affect what appears.
Place essential lettering away from the outside edges and consider where the avatar overlaps the header. Do not build a design whose meaning depends on a small logo in that corner. If your source already contains a slogan in the wrong place, return to the editable artwork; moving the complete crop cannot reposition the words independently.