Different resolution per layer?

Hi everyone, what I am trying to achieve is to import an image as reference (e.g. a tree) in full resolution (e.g. 2000x2000). Then I want a layer over this with a selected resolution (e.g. 256x256) so I have sort of a pixel grid over the reference image. I could then decide for every pixel in my reduced image how to represent it best and still compare to how my reference looks. Is this possible? If so, how? So far I have only managed to import an image and draw pixels either in its original resolution or scale it down, which obviously removes the details I want as reference.

Thanks!