I am referring to Photo. I have tried you suggestion and it does what I want. Display as DPI means when the image is projected on to a screen. When projected the surround is usually black. If the image is B&W there could be areas where the image merges into the background making it difficult to see the edge of the image. If this happens it's probably good to put a keyline/border on the image. Changing the background to black in UI allows me predict if the image will cause a problem when projected and I then see if the keyline(colour,thickness) will resolve the issue.
It would be useful if the background could be changed to any colour that would allow users to see the affect of using coloured mount boards for the printed images.
Many thanks for your help.