Dipper's Guide to the Unexplained

In Dipper's Guide to the Unexplained there was a flash of a picture at the end of each episode and if you put all the pictures together it makes this below.

Along the bottom theres a code, when decoded by using the three letters back method it says- FROM THE FIRST UNTIL THE LAST SEARCH THE CODES OF CREDITS PAST. ONE MEANS ONE SO SEARCH THEM ALL. WELCOME TO GRAVITY FALLS. 

Also, in "The Tooth" when the monster/ island comes out it speaks backwards. See it here-
.

It says "You've awoken me from my slumber. Enter my mouth children. Enter your destiny.

If you want to watch all the clips, heres the link for all of them in one video:
Dipper's guide to the unexplained 1-6