Template Canvas options -- Image on background, Opaque, manually setting index choices

I have a need to have template canvas, to be over either one big item in the canvas.

Possible ways I’ve found to do so would be:
-Background image on the template canvas (does not seem to be able to)
-Setting opaque on the canvas itself, resting over an image (I cannot seem to find an opaque setting, maybe a layout constraint (very little info I could find on layout constraints)
-Modifying the particular template that needs to be in the background, with a Z axis, and ensuring it is the Largest index.

The last option is an issue because I give the user the opportunity to dynamically add templates in the live form. The dynamically added templates are also interactive. I have that set up to get the resourceID of the template to interact with it, however, if the template index # is larger, clicking on the template will recognize the first index number it encounters.

I do not need the background image to be interactive, index being last is the last option, I’m hoping somebody has a better idea. Thanks.