Obviously, the "transfer them using iTunes" method isn't really viable.
I figured out where the logos go (/storage/<sd card>/Android/data/com.ootpdevelopments.iootp/files/AppData/prefs/saved_games/<saved game.lg>/news/html/images, and just overwrite the game-generated ones), but I can't figure out where to put ballparks so that they get picked up. I tried putting them in TarData_v36/data/ballparks which is where the default park images are, but I'm still getting the default ballpark image. Is there a specific file type naming convention I have to follow?