Right now the plugin only supports rectangular masking when using a bounding box, but there is workaround how to use an image as overlay for the whole product.
In this demo I am using that technique. I created an image that has the same dimension as the stage(canvas), but with a transparent area in the middle, that looks like a circle.
The white area is transparent
I added this image as initial image element in the Product Builder and enabled the option "Stay on Top" under the Modifications options. Whenever a customer adds an image to the stage, the image will be added behind the overlay image and he is seeing only the image part which is visible in the circle.
Of course you can create your own image overlay and upload it, e.g. you can even have a transparent area that looks like a Phone-Case.
Another alternative is to usk masks as described in this article.