You use png. That is not the right format for the task. Use it for pixel accurate things and for your developer master files. Not for distribution of regular images/illustrations.
Learn how to use webp. See the quality and size difference yourself. Your 3MB illustration files will be around 150-300kB without any visible quality loss. This is orders of magnitude. Renpy is made to ignore file endings. You can switch an image with no effort on coding.