Nostr is quite diverse so the aspect ratio could be anything. Typically you cannot go wrong with formatting images for mobile because a lot of us use mobile clients to read stuff. Wide screenshots are hard to see on mobile and you have to always zoom in. Tall images 2:3 work better on mobile but may take up more space on desktop.

It really depends on the subject of the image. A wide dashboard view is probably best presented in landscape, but an AI generated image will probably look best in the portrait mode.