Internal Image as Button

#!/usr/bin/kivy
import kivy
kivy.require('1.7.2')

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button

Builder.load_string('''
<MenuScreen>:
    GridLayout:
        cols: 1
        Button:
            on_press: root.val1()
            Image:
                source: "myimage.PNG"
                size: self.parent.width, self.parent.height
                allow_stretch: True
                keep_ratio: False
           
''')

class MenuScreen(Screen):
    def val1(self):
        print "i am executed"

sm = ScreenManager()
menu = MenuScreen(name='menu')
sm.add_widget(menu)

class MainApp(App):
    def build(self):
        return sm

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