Ahhh...I think I know. Just go to search with Ctrl + F, and search setPokemonBitmap. Replace lines of code which look something like this
with something like this.
Sometimes the sprite will be "pokemon3" (when in double battles) so you want to keep the sprite name the same, just remove the .setPokemonBitmap function and use the one specified. That should clear up the problem.
Code:
@sprites["pokemon1"].setPokemonBitmap(@battle.party2[0],false)
Code:
@sprites["pokemon1"].bitmap = pbLoadPokemonBitmap(@battle.party2[0],false)