Imageviewer and constraint

I don’t have too many issues trying to do the constraints for objects on simple screens but am struggling to to working this out for all devices.
I have a background image with a white rectangular region and want to get an image viewer to stay within the area and scale as the background picture does on each device. What is the approach to firstly anchor the Top and Left and then scale the image viewer to fill the area?

It is probably best to just use a canvas and draw the picture in that region manually instead of using an image viewer

Ok, but is it not possible to do it with constraints? What if I wanted it to be a button over that region or a textarea I would be back to the same problem, I can’t seem to pinpoint (x,y) across all the different devices to match the background.