Kind of odd, but I get it ... Put the toothpaste where you will want the nail to go. Then, press the picture to the wall where you want to hang it so that the toothpaste from the back of your frame sticks to the wall.
Really cute idea for jazzing up old file cabinets with picture frames. (scheduled via http://www.tailwindapp.com?utm_source=pinterest&utm_medium=twpin&utm_content=post1492495&utm_campaign=scheduler_attribution)