Firstly to not use a BG just uncheck the BG Image box on the BG Tab, now the BG will be the Ambient Depth colors and whatever Fog you use. There is 3 ways a BG can be loaded, Normal by just checking the use BG image Box. The image will be loaded as an image(bear in mind the aspect ratio of both the BG being used and the Image being rendered) if you load say a square 1:1 BG image and using a 4:3 or 2:1 or whatever aspect ratio for the rendered image that BG will be strecthed to fit, unless it's an abstract of some sort it is generally better to use a BG of the same aspect ratio as the image being rendered to minimize distortion of the BG. The 2nd load option is Scale Rows to Geographic Projection which basically stretches the top and borrom of the BG along the Horizontal. The 3rd option is load as Full BG Sphere, this spherizes the BG and now you can use the X.Y,Z sliders to rotate the BG on the Axis. The smooth on image load just adds a smoothing filter to the BG Image on loading and the Add to BG Depth (not blend) determines how heavily the Fog (Depth and Dyn) react with the BG.

Remember...No matter where you go there you are