is a keyboard binding functions .

The function waits for specified milliseconds for any keyboard event. if you press any key in that time the program continues. 

if 0 is passed it waits indefininetly for a key stroke.

you can create a window and load image to it later. in that case you can specify wheather window is resizable or not. This is done via the function cv2.namedWindow()

by default the flag is cv2.WINDOW_AUTOSIZE , you can specify flag to be cv2.WINDOW_NORMAL

cv2.namedWindow('image', cv2.WINDOW_NORMAL)

cv2.imshow('image', img)  { remember image named as to be the save as above

Use the function to save an image

cv2.imwrite('example.png', img)