Change the design in bounding box

Can anyone give me some advice ??

I would like some advice on how to configure this solution.

I've created two levels of element editing with a bounding box:

1) text: perfect works the user can modify the text at will inside the bounding box.

2) already loaded design that the user can move to enlarge, BUT CANNOT CHANGE with another graphic present in the DESIGN category.

How should I do to allow to change the graphics that I have set by default with one present in the DESIGN category?

This is the page of the product to be modified:

https://www.masterlaser3puntozero.it/21/prodotto/boccale-da-birra-inciso-personalizzabile/