import re

def cloudrepl(s, i, o):
    def repltags(m):
        tags.append(m.group(0))
        return '#THEREISATAGHERE#'
    def backtags(m):
        return tags.pop()
    tags = []
    result = re.sub(r'(<img[^>]+\>)', repltags, s)
    result = result.replace(i, o)
    tags.reverse()
    result = re.sub(r'#THEREISATAGHERE#', backtags, result)
    return result

print cloudrepl('<img name="whatever" src="/your/friends/whatever" alt="whatever"/> whatever \\"whenever\\" <img name="whatever" src="/your/friends/whatever" alt="lol"/>', 'wh', 'aa')