Open a drawing program (kolourpaint, mspaint) and pick a pixel brush, then run the script with an image as the argument. You'll have 2 seconds to place your cursor on the top left of where you want the image. You can change the SQUARE_SIZE env variable to change how big a cell is. Make sure that that image size times SQUARE_SIZE isn't too big or it'll begin trying to draw outside your screen which isn't ideal.
In case you wan't to escape its hold, pyautogui stops if you move your mouse to the top left corner of your screen.
Input image:
Output (with size 9):
