My background image is set to -10 on the z position. The image is 800x600 and I set the pixels per unit to 50. I am running out of ideas as to what it is.
I think you have the z positions of the camera and the image backwards. I think the camera is suppose to be at -10 and the image at 0. If you click on the 3D button on the scene view you should be able to verify this.