import re, os, Image

re_file = re.compile(r'(?i)(.*)(\.jpg|\.gif|\.png|\.bmp)$')
thumb = '-thumb'

for fn in os.listdir('.'):
　m = re_file.match(fn)
　if m and os.path.isfile(fn) and not m.group(1).endswith(thumb):
　 　r = Image.open(fn)
　 　r.thumbnail((200,200), Image.ANTIALIAS)
　 　fn_new = m.group(1) + thumb + m.group(2)
　 　print fn, '->', fn_new
　 　r.save(fn_new)