Return a Google Images Search URL for a given search term.

def _search_url(search_term:str, size:str='>400*300', format:str='jpg') -> str:
    "Return a Google Images Search URL for a given search term."
    return ('https://www.google.com/search?q=' + quote(search_term) +
            '&espv=2&biw=1366&bih=667&site=webhp&source=lnms&tbm=isch' +
            _url_params(size, format) + '&sa=X&ei=XosDVaCXD8TasATItgE&ved=0CAcQ_AUoAg')