Builder.load_string(
    """
<ButtonsApp>:
    orientation: "vertical"
    Button:
        text: "B1"
        Image:
            source: 'kivy.png'
            y: self.parent.y + self.parent.height - 250
            x: self.parent.x
            size: 250, 250
            allow_stretch: True
    Label:
        text: "A label\"
"""
    )


class ButtonsApp(App, BoxLayout):

    def build(self):
        return self
Passing image object as a button background in Kivy