The background image is mapped to either DOME GEOMETRY or CYLINDER GEOMETRY depending on this setting. This geometry shape surrounds your model, but cannot be directly manipulated or rendered. You must use the 3D Placement options (see below) to transform it.
Click the Map button next to this field to choose a file texture containing the environment image you want to use as your background. This should be a large panoramic image in tiff, jpg, gif, bmp, or png format.
This slider rotates the environment geometry texture around the Up (vertical) axis.
This slider proportionally scales the environment geometry texture.
This slider moves the environment geometry texture to a certain height above or below ground level (Z = 0). Clicking the Auto button automatically sets the base of the environment geometry to the bottom of the model's bounding box.