Thread:ヌヌハラ/@comment-76.183.188.242-20200822055749/@comment-44256294-20200822152834

There is a code at the end of the file which indicates another image to use as the background. The game engine will make the magenta color transparent and automatically compose the two images.

What I've done in the past is to manually combine the images, save as .png and then replace the .webp with the modified .png in ALDExplorer. I haven't reverse engineered it enough to know how to rebuild these special .webps, as I believe there is a whole section of data at the end, not just the single CG number.