The image generator in ChatGPT does not have the ability to produce the images in a different file format. You always get the download in webp. That seems be an item of contention in the past, but I think people have gotten used to it.
Producing a wide image instead of a square is an option though.
You can fill out one of the text boxes in “Customize ChatGPT”, under settings, to alter the behavior. You can use the box for how you want ChatGPT to act.
You can add a line that says something to the effect, “When you use dall-e to make an image, I always prefer the wide size by default.” That way, you don’t have to ask each time.
I hope that works for you!