Passing image object as a button background in Kivy

In addition to toto_tico's answer, you can locate the image at the center of the button as:

Button:
    id: myButton
    Image:
        source: "./buttonImage.PNG"
        center_x: self.parent.center_x
        center_y: self.parent.center_y

The source is just a property of Button and it is a string as you pointed out. You want a Widget inside a Widget, and that is the basic way Kivy works. So just add the Image as it is. A little bit of positioning would do the rest.

You have to be careful with the positioning. Make sure it is in a visible part and nothing covers it. I use a Label after the button because it has transparent Color so you can experimenting with it. For example if your positioning is wrong (try x:0 y:0) you can see the button going to the bottom-left corner in the label area.

The image I am using is the Kivy logo:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder

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

if __name__ == "__main__":
    ButtonsApp().run()