Thanks for reaching out to us!
In our desktop program or web version, you can achieve this by uploading your 2nd photo to the Photo Basket (at the bottom of the editor) and then dragging it onto your main photo.
On our mobile app, you can achieve this by choosing the “Watermark” option from the menu and selecting your 2nd photo.
Hope this helps! Let us know if you have any further questions.
– Regards, The Ribbet Team