GoPop is a free-to-download animated GIF app that helps your layer one GIF or image on top of another. The way that this works is the first image or GIF is displayed on the screen of your mobile device and the second is revealed when you touch the screen. The app is optimized for smartphones but it also works on a computer the same way when you click and hold an image.
The quick switch between the two images has been dubbed a "pop," which is where the app gets its name from. For starters, the GoPop app has a number of GIFs for you to get started with, although it's also possible to create your very own "pops" from scratch.