FILE_EXTENTIONS = 'png', 'jpg'

files = filter(
    None,
    (p.get('files') for p in req['threads'][0]['posts'])
    )

prefix = urli.rsplit('/', 2)[0] + '/'

paths = (
    prefix + path
    for path in (f[0]['path'] for f in files)
    if any(map(path.endswith, FILE_EXTENTIONS))
    )

for p in paths:
    label1['text'] = 'download ' + p
    # ...