main.kv

#:kivy 1.0


TestApp:    
    Image:
        source: 'moonphase21.gif'
        id: img

main.py

from kivy.app import App
from kivy.uix.image import Image


class MainApp(App):
    def build(self):
        return Image(source='moonphase21.gif')


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