Doesn´t Photoshop has those "Difference" filters, that show you the difference between the background layer and the next layer in colors? Black areas are then the same color as the background = no change, and every color other than black is a different color than the background has (and this would catch shadows too).
I just have Krita, and there they exist too, but my lack of XP with Krita hinders me in converting this difference layer then to a simple black and white filter mask, that you could use as to cut out the affected area for overlays.
But if you render out the pure background images, that you know will be used more often, then you could use them against those scenes, where different chars appear, and the diference layer should automatically give you the affected area, which you could use for overlays.
An example image of Kritas Difference filter at work:
Wait WHAT
I didn't know there was a Difference filter, that would make my life significantly easier!
I'll check it out rn
Ok, after some trial and error, i even found a combination of filters, that allow Krita automatically to create those masks for me - as in, i just need to drag in a background image and the image, i want to get the difference as an overlay, and it now automatically cuts out this area (thanks to the difference filter as fist step), then a recoloring filter recolors those differences to a black and white mask, then a clone of the image from which i want the difference is multiplied with that black and white mask, to get the color of this area, and the rest stays pure black. And the last step was just a filter to convert a defined color (black in my case) to transparency. or maybe change the order, so that you already convert the black and white mask to transparent and white, then multiply that with the color - that way even black in the difference will be kept and not accidently converted to transparency.
Now its just dragging in images and saving the result as a new overlay for background images.
I have tested it with some of your images, all in all was able to reduce the size by half for those, granted that i had no clean backgorund and needed to cover old chars too.
Your biggest problem is, that many images do indeed have slight camera movements, which ruin this, but there are still enough images, where just the chars poses change (and some anti aliasing "errors"), for which this still can make a significant difference.
