After taking optional photo #4 you will now want to make your way into the diner. Once you have made it into the diner, turn left and make your way to the bathrooms. When you have made it into the bathroom, look at the mirror above the sink and you will see writing that says "Fire Walk With Me" carved into it. Look at the writing and you will be prompted to take a photo. After you have taken the photo the achievement will unlock.