Thanks for reaching out to us! Unfortunately currently there isn’t a way to do it, but I do have a workaround for you: Add one of the circle shapes from the 4th sticker group “Photo Licenses”. Then use the Eraser tool on the Sticker Properties dialog that appears to delete the middle of this shape – you’ll be left with a circle that’s hollow.
Let us know if you have any further questions!
– Regards, The Ribbet Team