Follow the guide below to allow the customer to upload images to the back or front sides of the lightbox sign customiser.
Navigate to the admin of Sign Customiser for your store
Click on "Customisers" in the main menu on the home page
Click on the customiser you wish to edit
Click on "Lightboxes" in left hand the left-hand side menu
Click on the "face" or "back" sides of the shape to edit
Scroll down untill you see "Allow image uploads" and make sure its ticked.
Click save at the bottom of the form.