Because I use TTS, I've noticed with every animation you have, there is a missed image listed. Without the TTS I would not know this. I think I have figured out the cause. The image lists a different number at the end. My guess, the game is looking for one more image than you have, or you are not starting the numbering from 0.
Sorry for making this difficult to understand. My brain is out to lunch, to see what I'm talking about press 'V' when in-game to activate RenPy's TTS.
Hope the above is understandable.
PS Cool on using Grammerly.