如何在kivy中改变按钮的背景图像？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout

class MyBoxLayout(BoxLayout):
    def __init__(self, **kwargs):
        super(MyBoxLayout, self).__init__(**kwargs)
        
        # 创建按钮并设置背景图像
        self.button = Button(text='Click Me',
                             background_normal='normal.png',
                             background_down='pressed.png')
        
        # 将按钮添加到布局中
        self.add_widget(self.button)

class MyApp(App):
    def build(self):
        return MyBoxLayout()

if __name__ == '__main__':
    MyApp().run()