class Myszka(Widget):

    def on_touch_down(self, touch):
        ...
        App.get_running_app().root.add_widget(self.flaga)

import os
from kivy.app import App
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.image import Image


class Mouse(FloatLayout):

    def on_touch_down(self, touch):
        img_path = "/home/iam/Pictures/AppImages"
        flag_path = os.path.join(img_path, "Android_celebrate.png")
        flag = Image(source=flag_path, size_hint=(None, None), size=(64, 64),
                     pos=(round(touch.pos[0], 1), round(touch.pos[1], 1)))
        self.add_widget(flag)


class TestApp(App):

    def build(self):
        return Mouse()


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