Hi Vanessa, Thanks for contacting us and for your excellent suggestion. We’ve added this to our wishlist for inclusion in a future Ribbet version.
Although we don’t have a Chromakey feature right now, we do have an eraser tool which you can use to achieve similar results (although manually). Start by opening up the background image, and then drag on your foreground image from the “photo basket” at the bottom of the editor. You can then use the eraser to delete areas from the foreground photo. You can also watch a demo video of how it works here.