I'm new to this community. My First answer that you deleted "The ultimate secret of this puzzle is nice.", provided enough evidence to the author that I soved his/her final secret, and respected "Please keep further secrets secret". Anyway....
LB Lenna to:
Overlay both images in "difference" mode and you get "Congratulations! You have found the ultimate secret of this puzzle"
Let me know if you are unhappy with my answer again.

