import os
from random import randint
from shutil import copyfile

# create array item for each jpg in the logon folder
jpgs = []
for root, dirs, files in os.walk(r"C:\Logons"):
    for file in files:
        if file.endswith('.jpg'):
            jpgs.append(os.path.abspath(os.path.join(root, file)))

# select random array item to be copied to system32\oobe..\backgrounds
copyfile(jpgs[randint(0, jpgs.__len__() - 1)],
     r"C:\Windows\System32\oobe\info\backgrounds\backgroundDefault.jpg")