Currently the only available aspect ratio is 4:3. By turning custom images beforehand 3:4 is also possible. An option for 16:9 and maybe even more aspect ratios would be really nice.
I know that you can still use images that are not 4:3 but the stretching and shrinking sometimes isn’t that nice
Check out this mod:
https://steamcommunity.com/sharedfiles/filedetails/?id=2685357825
It has a variety of aspect ratios, both portrait and landscape.