You're correct, the models are animated using Live2D! In theory you should be able to modify the image in
game/images/Models/Student_1/Student_1.4096
and the change will be reflected in game. Due to how Live2D handles the image only changes close to the silhouette of the original art will work, and you'll need to keep the image size the same.
I occasionally release the Live2D models as a modding resource as well, so if you want to fiddle around with those they're available
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. I'm just now realizing I haven't released an updated pack since v0.17; I should probably put out a new version soon.
If you're interested in coding stuff
A Very Full House has some modding tools in the code that let you define image changes for models dynamically. That's a little much to explain right now, but there's some example code in
game/Mods/example_mod.rpy
. We've also got a mod development channel
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where I'm happy to answer any questions you might have!