First of all which ao oni are you playing? It could be just a version issue.
It could also be that you can't use "key 3" (whatever that is) when you find it. In that case, I don't know how that can be fixed until the RPG XP player updates...
6.23? I never knew you could play that version of Ao Oni on Neko player. How do you get it to run?
Anyways, if the PNG is not set to the icon, you'll probably have to set it as an icon. How you do that is beyond me (guess you'll have to google it; the answer must be somewhere online). If I could play the game, maybe I could also run into the same problem and try to see if I could find a fix. Can you try to send me a download of the game?
It could also be that you can't use "key 3" (whatever that is) when you find it. In that case, I don't know how that can be fixed until the RPG XP player updates...