You can enable your Main Client and Photographer Invited users of the gallery to upload their own photos to be displayed in the gallery itself.

From the Client Gallery's Gallery Setup tab, checkmark Client can add photos in the gallery.

This will create a My Photos scene at the bottom of the gallery. Clients can click the Upload button to add their own images.

They will also be able to purchase products from the shop using these images, unless disabled in your Preferences.