I've asked this before and someone told me that, by now, it's not gonna happen.
This is because in early-development-stage, Hyao (the creator) asked if we would prefer the main charact to have a skin or not.
Most people choosed to not have a skin and this is why the MC have no sprite and probably won't have.
I code a bit in my free Time and in my opinion adding such an option would be doable but... If I "need" to create an in-game interface for "implementing" an image into my game files images folders... that would requiere a LOT of ajustments to make it into a valuable implementation. It CAN be done but it sounds kinda tiresome to code 😓.
Not even talking about the fact that there is a lot of things to take into consideration concerning the mc's images format, size,...
