A short, wide strip — not a post image
How X crops the header
X displays 1500×500 at upload but overlapping UI (avatar, buttons) covers the bottom-left. Keep the subject in the upper two-thirds and away from the left 400px on mobile. A face at the bottom-left will sit under your profile photo.
Not LinkedIn, not Facebook cover
LinkedIn cover images are closer to 1584×396 (4:1-ish). Facebook covers are wider and shorter depending on device. Export a dedicated file per network. 1500×500 is the X header job.
Text on headers
Tiny slogans vanish on a phone. If you need a slogan, put it in the bio. This resizer only changes pixels.
After upload, X recompresses
Even a perfect 1500×500 JPG may gain artifacts. Start from a clean graphic, export JPG, skip PNG unless you have hard-edged brand shapes.