I cant say that this solution is pretty elegant, but it's short and working. But... platform dependent, because of IE. Anyway here it is :
from win32com.client import Dispatch
from time import sleep
from sys import exit
IE.fullScreen = 1
while (IE.ReadyState != 4):
img = ImageGrab.grab((0, 0, 1440, 900))
Code is pretty simple- it launches IE in full screen mode, waiting while page being loaded, makes screenshot of whole screen resolution - 1440 x 900 in my case, and happily exits.