You have to be careful adding sprits, some are just to big/small and look a little strange. As far as I know, there isn't a way to edit the sprites. You can however, if you google for some sprits and look closely, find the ones used for the default of the system. And if you know how (I don't) you could edit them from there.
Sorry! You may want to just keep trying, I'll post some sprites I've found that work pretty well for me sometime relatively soon.