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, desktop trims top and bottom — so no placement survives everywhere. Two rules cover it:
- Keep what matters in the top two-thirds, centred or right. On mobile, the profile photo and bio overlay the bottom strip — it's mostly hidden.
- Bottom-left is always occupied by the profile photo (roughly 420×160 of the frame). Nothing important goes there.
Fitting a photo to 3:1: the ratio is severe — most landscape shots lose their top and bottom thirds. The crop position slider picks which band survives. Panoramas, skylines, wide scenes and textures take the shape well; detailed group photos mostly don't.
Format choice: JPG for photographic headers (smaller), PNG for graphics with text needing sharp edges. Output here is JPG, auto-fitted under the limit, real final size shown per file.
Drop several candidates and compare before uploading. The crop and encode run in this page on your device — nothing is transmitted, and the only upload is yours, to X.