It won't be simple. I tried to find a plugin which can combine layers into film strip, but unsuccessfully.
I think it was a gif initially, it was you who pointed out that we can just convert gifs to filmstrips. I tracked the issue down to death effect with dissolve, my animation class somehow doesn't like when dissolve is running at the same time as it. Spells/Death all work really well now, I am going to test them on faster PC soon just to make sure. Maybe a rewrite of Animation class is also a good idea, we could simply force it to show correct images at the correct time instead of trying to time them using renpy.redraws, it's not too hard and should solve the animation overshoot issue (I just hope that it will not create a new one where frames are skipped, but even if it does, that will be a lot harder to notice).
In any case, I am still testing, tweaking and updating some spells. I wanna try projectile spell that hits three targets with individual attacks, that should be possible now
On the other hand, plugins for gimp are made in python, so you probably can write one by yourself
I didn't know that but I don't have Gimp atm...