The list of buttons is in:
Create your buttons using your favorite image editor. The buttons may to be saved as GIF, JPG or PNG. If you are not using the same format or name as the original buttons, edit the button_names.php and save the edited file in your language override directory.
Upload your button images to:
NOTE: If you are using multiple languages you need a set of buttons for each language.
See: What directories contain all the images? (i.e. buttons, icons, images etc.)?