You can create a custom sticker by following these steps:
1. Create a Blank Canvas and tick Transparent background
2. Create your sticker using any elements from the editor, or you can also upload your own elements to the Photo Basket at the bottom of the editor
3. Save your image (sticker) in PNG format
You can then upload this sticker back to the Photo Basket at the bottom of the editor, and use it on any photo. It will maintain its transparency.