X (Twitter) headers: 1500×500 pixels, 3:1, JPG or PNG, under 5MB. Drop an image; it comes back at exactly that spec, cropped where you choose.
X re-crops headers per device: phones trim the sides while desktop trims top and bottom. No single placement works everywhere, and two rules are worth following:
- Keep what matters in the top two-thirds, centred or right. On mobile the profile photo and bio sit over the bottom strip, and most of what you put there will not be seen.
- Bottom-left is always occupied by the profile photo, roughly 420×160 of the frame. Nothing that has to be read belongs in that corner.
Fitting a photo to 3:1: the ratio is severe enough that a typical landscape shot loses about half its height, and the crop position slider picks which band is kept. Panoramas, skylines, wide scenes and textures take the shape well; detailed group photos mostly don't.
Format choice: JPG for photographic headers, which come out smaller; PNG for graphics with text needing sharp edges. Output here is JPG, auto-fitted under the limit, with the finished size shown per file, and several images can go through together so they can be compared at header size before one is chosen. Arriving at exactly 1500x500 also leaves X no scaling to do, which keeps logo edges and lettering crisp instead of softened on the way in.