Best way to use large hand-drawn images for 2D mobile game?

Hey Everyone!

So this may be more of an art question than a "Unity" question, but I'm relatively new to this and I'm not an artist so I need some help :)

I'm currently working on a 2D runner game where the character is running on a sphere that is constantly rotating. Right now, I have it so the camera is zoomed in on the top of the sphere so it's only showing a small portion of it at a time, most of the sphere is off camera. I feel like there's a way better way to do this, but this is where I'm at now.

However, my problem right now is dealing with image size. The artist I'm working with is using hand-painted watercolor images for everything. The sphere I'm using right now is 1024x1024 and taking up a massive 16MB of space! Granted, I have it at max image quality right now, I realize I will most likely have to lower this. I'm just trying to find the best way to save space, but keep the quality as high as possible.

Target platform is iOS so I want to keep the file size as low as I can. I know that may be difficult with this art style.

Any help would be much appreciated!

Thanks! Jordan