Hi Ic4p,
Both the promotion piece buttons and the flip-board button are implemented using Unicode textbuttons. There is a flip-board button image in the asset folder but it isn't used. (See the code linked below.) If the Unicode characters aren't showing up, you would want to replace the textbuttons with image buttons and use image assets. If you have any more questions, please feel free to reach me on Discord. My handle is lynn#9327.
https://github.com/RuolinZheng08/renpy-chess/blob/bb3eeac7ec2bb5d2728d161db29388...
https://github.com/RuolinZheng08/renpy-chess/blob/bb3eeac7ec2bb5d2728d161db29388...