It's mainly a space-saving trick, @Larequirem, where they separate out the static and dynamic parts of an image so that they can just overlay each dynamic part on top of the appropriate static part. It saves space because the dynamic part is typically smaller.
...What weirds me out about it is how _inconsistent_ the gallery is about it, honestly. They mainly seemed to use it for about half of the faces, and for the girl's breasts in 847-858 (compare her outfit's details to 846, and her posture to 878-885, and it very much hints that her boobs are in pics 878-885, and the rest of her is in 847-858), but there were a LOT of other places they could've used it.
@conundrum I get why they do that in these games, but it would be nice if it was uploaded properly. This happens when uploaders just take the images straight from the folders because that's just how they are. They're pieced together in game, right? Instead of piecing them together manually, can't they just get a full save and screenshot each image in the CG recollection room where they're already pieced together? There might be an unwanted background, but better than faceless and quicker than manually doing it.
Well, @Larequirem, at least this one's uncensored.