The i parameter would make it possible to dynamically pass an image stored in an external gallery into a product image area.
An example can be seen in the below url:
To expand on what the link is showing - the front end selection of the 'design' is essentially a reference to an external gallery image. When selected, that image is then being automatically applied to the products image area when the app loads.
This means you could have a front end selection for the user to pick a product(thumbnail), that could then call the related gallery image and apply it to the back of the card when the App opens. This image could include the logo and any text as required.
In terms of the requirements of the product setup, the key for this to work is to ensure that the 'product Image Area' on the card is the very first image area in the list. External images get applied to the first image area by default so this is why it is important.
And using the i parameter this can be applied as :
Any question about this please contact firstname.lastname@example.org