How you position a texture or image file background depends on whether the background’s Image Plane attribute is Attached to Camera or Fixed.

When you create a background for a perspective camera, Image Plane is Attached to Camera by default. When you move the camera, the image plane also moves. You use this setting when you are using the image plane as a reference for animating objects in the scene (that is, motion matching or rotoscoping), or if you plan to render the image plane as a background.

When you create a background for an orthographic camera, Image Plane is Fixed by default. When you move the camera, the image plane does not move. You use this setting when you are using the image plane as a reference for modeling objects in the scene.

For more information on image file backgrounds, see Image file backgrounds.