Mirage class used for special effects inside BE enforces background resizing.
class Mirage(renpy.Displayable):
def __init__(self, displayable, resize=(1280, 720), ycrop=8, amplitude=0, wavelength=0, **kwargs):
It's fine only as long as battle background is 1280x720 already. If it's not, then before animation appears you see abrupt change in size, and then it jumps back to initial size after the animation. It looks terrible.
I'm not sure why you have to resize something before miraging it, but we need a decision here: either you make it universal or I resize all battle backgrounds.