Add method to existing class on the fly

from psychopy import core,event,visual
def rescale(self, width=0, height=0, operation='', units=None, log=True):
    (old_width,old_height) = self.size
    if all([height,width]):
        pass
    elif height:
        ratio = height/old_height
        width = old_width * ratio
    elif width:
        ratio = width/old_width
        height = old_height * ratio
    self.setSize([width,height],operation,units,log)

visual.ImageStim.rescale = rescale

win = visual.Window()
img = visual.ImageStim(win,image='pic2.jpg') # point this to an image you have locally.
img.draw()
win.flip()
core.wait(1)

img.rescale(width=.7)
img.draw()
win.flip()
core.wait(1)