My maps are offroad maps with white border with coord info and some date and producer detail...
So the white border covers an area already described by another map which i have already converted in another tile store folder....

I'm sorry but i can't understand the purpose of
ImageChops.lighter(im1, im2) where im1 im2 are pieces of maps.....
maybe that your images are black with white lines?

Now i'm writing something to compare the size of the tiles before overwrite (the white one is smaller)