Single images on X: 16:9, ideally 1600x900, under 5MB. Drop an image and it comes back at exactly that, cropped where you choose.
Why 16:9 matters there: the feed shows landscape images full-bleed with no trim, and a 16:9 upload is the one shape that arrives exactly as composed. Any other ratio gets a feed crop you don't control, and the part of the photo you cared about is often the part that gets trimmed.
Why 1600x900 and not smaller: 1200x675 circulates as the standard and is the same ratio; the larger size simply survives high-density screens better. Bytes stay modest because the tool fits the JPG under X's 5MB image cap automatically, though a photo this size rarely gets near it.
One image, this shape. Post two, three or four images and X switches to its collage layouts with their own crops; this page sizes the single-image case, which is the one you fully control.
The crop slider picks which band of your image survives the 16:9 letterbox. Skylines and wide scenes settle in easily, while tall portraits rarely do, and it is better to choose a different crop than to accept one that cuts across a face.
Drop several images together and each comes back at 1600x900, reporting its final size.