File > Import > Image Plane

Imports a two-dimensional image into a camera.

You can use image planes for the following:

Rotoscoping – Using live action footage in the background to which you can match your models.
Model Guides – You can set up three orthographic drawings as matte planes to use for model building guides.
For example, in scenes with many objects, you can pre-render the objects that you are not working on and use these renders as matte planes.
Sprite Animation – You can animate matte planes as you do 3D models.

An image plane is associated with the camera.

Import Image Plane Options

Size: Fit window – Fits the image plane into the modeling window.

Specified – Imports the image plane at a specified size.
Horizontal: If you select a Size of Specified, the Horizontal option appears below it so that you can specify the horizontal width of the image plane in centimeters.
Display Mode: RGB shows a color preview of the image. Box shows only an outline for faster display.
Depth Type: Determines which image planes appear in front of each other or the wireframe model. This setting is Off or Priority.

When it is Priority, a slider is also displayed, which determines which image planes appear in front of each other or the wireframe model.

Image planes and the model are drawn in this order, from back to front:

Image planes with Depth Type set to Off

Image planes with Depth Type set to Priority and Depth of 0 or less, from lowest to highest depth

The wireframe model

Image planes with Depth Type set to Priority and Depth greater than 0.0, from lowest to highest depth

An image plane with depth greater than 0.0 appears in front of the model, whereas other image planes appear behind the model. Depth drawing is in sorted order so an image plane with a depth of 2.0 appears behind an image plane with a depth of 3.0, and an image plane of depth -3.0 appears behind an image plane of depth -2.0.
Crop to Window: If Crop to Window is ON, the image plane is cropped to fit in the selected window. By default, this toggle is turned OFF.
Dim Image: If Dim Image is ON, the image plane is greyed somewhat to create an inconspicuous background. By default, this toggle is turned OFF.