Here's a snippet of the Guardian Recipe.
It does have the masthead_url defined as you can see below.
Code:
class Guardian(BasicNewsRecipe):
title = u'Custom The Guardian and The Observer'
if date.today().weekday() == 6:
base_url = "https://www.theguardian.com/theobserver"
cover_pic = 'Observer digital edition'
masthead_url = 'http://static.guim.co.uk/sys-images/Guardian/Pix/site_furniture/2010/10/19/1287478087992/The-Observer-001.gif'
else:
base_url = "https://www.theguardian.com/uk"
cover_pic = 'Guardian digital edition'
masthead_url = 'http://static.guim.co.uk/static/f76b43f9dcfd761f0ecf7099a127b603b2922118/common/images/logos/the-guardian/titlepiece.gif'
__author__ = 'Kovid Goyal'
language = 'en_GB'