Draw Image as a Background...

I’m working on an AR project and I’ve run into some difficulty solving a specific problem.

Basically, I have an image and I want to draw that image to the screen, as if it were a UI element, then I want to draw everything else on top of that background. I’ve tried placing a plane inside the scene and applying a material to that plane, but, the results aren’t very pleasing. If the element is drawn as a UI element, it draws after the scene and over the models in the scene.

Any suggestions?