I have used that program quite a lot. I believe the problem you are having is that in the scale menu the "link" between the horizontal and vertical size is enabled. You can click on the "link" icon (it looks like a chain link). I would suggest that rather than scaling the image (it will distort it), that you change the canvas size to 960x800, then you can center the image as you like. After that, just "Resize layer to canvas." All of these should be in the "Image" and "Layers" pull-down menus. If you're still stuck, I can go into it and be more specific.