from os import listdir, rename
from os.path import isfile, join
from random import choice
from string import ascii_lowercase, digits
mypath = 'c:\\1'
l = 6
onlyfiles = [f for f in listdir(mypath) if isfile(join(mypath, f)) and f.upper().endswith('.JPG')]
for f in onlyfiles:
  rnd = ''.join([choice(ascii_lowercase + digits) for _ in range(l)]+['.jpg'])
  rename(join(mypath, f),join(mypath, rnd))