Annoyingly, I can’t seem to get Bing to generate an image that isn’t square.

My experience with Stable Diffusion – which is trained on square images – has been that one is generally better-off generating square images to get an initial scene, and then cropping, upscaling (something that generative AIs do well and I’d guess that Bing probably can do, though I don’t know for sure), and possibly outpainting as a way of getting more pixels and the aspect ratio desired.

Non-square images have been more-prone to things like weird mutant monsters with lots of legs, though I did try a run with my current model (based on SDXL) on a non-square image and it seemed to be working all right, so I don’t know if things have improved here or if I just got lucky.