TransitionScreen.kv

<TransitionScreen>

   name: 'TransitionScreen'
   pratik2: pratik2

   FloatLayout:
      size_hint: None, None

   ImageButton:
      id: pratik2
      source: "tiny_pratik2.jpg"
      size: 100, 100
      x: root.width * 0.5
      y: root.height * 0.05
      on_press: root.transitionback()