Replace layer by another image

Hi!

I have 3 layers in my product: background image, "container" phone case, and upload zone.

I want to "replace" source image for background image and refresh my view... Change one layer of my product for another image...

How can I do this?

I have this JS, where thumbnail is just a new image url.

fabric.util.loadImage(thumbnail, function (img) {
 var fondo = fancyProductDesigner.currentViewInstance.getElementByTitle('Fondo');
 if ( fondo ) {
  fondo.setElement(img);
  fancyProductDesigner.currentViewInstance.stage.renderAll();
  fondo.bringForward();
 }
});

Issue in this code is that my "fondo" ksclass bring to top hiding all elements of my design.

How could you achieve this funcionality? Change a layer url image for another one with no change in the rest of elements-