helderres, I import backgrounds all the time, and my set up, except I have an nvidiaGTX560, is identical to yours. Set the preview window for 1200 x 800,  import the 2400 x 1600 size and the image will load and fit fine. Render at the 2400x1600 size. Oh I also undock my preview window when I load the background to be sure it covers the entire area. Once you are assured of that redock, and just add your characters and props, position and render.

I always split the image size in half for the preview dimensions (*you still load the full size). Be sure to uncheck the "constrain aspect ratio" until you reset the render size, then recheck it and save for this render. See the screen shot for info on the corrections! ;)  I hope this helps you!

~A~

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