- The top and the bottom. X says it may crop up to 60 pixels from the top and the bottom of a header, depending on the screen and the browser. The resizer stripes those bands.
- Your profile photo. On a computer it sits at the bottom left, a little in from the edge, with its middle on the header’s bottom edge. Its top half covers that corner of the header. The resizer draws the circle there.
- Phones. The header shows smaller, so fine print gets hard to read. Big shapes and short words work best.
Put faces, logos and words in the middle third, away from the bottom left, and the header reads the same everywhere. Bluesky crops its banner to the same 3:1 shape, so one picture works on both.