The overlay (or layer) eraser tool lets you erase out parts of an image so it blends with another image exactly the way you want it to. This tool is useful when combining two photos or when combining a photo and an overlay.
Using the eraser to blend an overlay with a photo:
Start by clicking “Edit” on the home page, and opening a photo.
Using the eraser to blend two photos: Follow the same steps above, but instead of choosing an overlay, click the “Your Own” button atop the panel, and upload your second photo. That photo is your overlay, in this case.