To achieve this:
1. First open up the new background image
2. Upload and drag on the main image from the Photo Basket (at the very bottom of the editor)
3. Click the Eraser icon on the Photo Properties dialog that appears
4. Paint over the area of the main image you want to erase
We have a brief demo video illustrating this here.
Let us know if you have any other questions.
– Regards, The Ribbet Team