Doesn’t matter which format bmp, jpg, png …
Files can be read from, but (as far as I’m aware) there’s no pre-existing code for reading from any common image format. That’s no reason to say they couldn’t be added, though I think it would be better to use a non-standard format to minimise code wasted on superfluous features (i.e. stuff the Pokitto wouldn’t need).
Happy to be proven wrong.