Hi, Thanks for contacting us!
In order to use a custom sticker, simply upload the sticker to the Photo Basket at the bottom of the editor. You can then drag it on, resize it, and use it as a sticker.
In order to maintain transparency in your logo, first create it in Ribbet using a Blank Canvas with the transparent background option ticked, and then save it in PNG format (from the format dropdown on the Save tab). Then, when uploaded to the Photo Basket and dragged on, your logo will maintain its transparency (and won’t have a white background).
Let us know if you have any further questions!