Resize an image for X

For an in-timeline post, 1600 pixels on the long edge is past the point where more helps: X downsamples and re-compresses anything larger, so the extra pixels are thrown away along with some quality. What matters much more is the shape. A photo in an unexpected proportion gets cropped by the platform to fit the timeline, and the platform picks where - preparing it here means you do.

Because it is a different surface. The card shown when someone shares a URL uses a wide summary image, and a square photo placed there gets its top and bottom taken off. If the image is meant to be seen on a link rather than in a post, use the card option - it is the one place where getting the proportions wrong reliably cuts a face in half.

No, and it usually gives slightly worse. X re-encodes what it receives. If you upload something far larger than it displays, it downsamples and compresses in one step of its own choosing; doing the resize here first, carefully and once, generally survives that better than handing it a 12-megapixel file.

Yes. The preset strips metadata, so the GPS coordinates a phone records are not carried into the file you post. That matters on a public timeline more than almost anywhere else - a photo posted from a camera roll can carry the exact spot it was taken. The metadata viewer will show you what was in the file first.

No, and it is worth stating plainly: the crop, the resize and the metadata removal all happen in your browser on your device. Your photo reaches X from you rather than from an intermediary, and a content security policy the browser enforces prevents this page from sending it to another domain.