You can choose to display your free image plane in all views; or, only through a specified camera. The drop-down list beside the Display > looking through camera allows you to select the camera through which the image plane is displayed.
You can also specify the camera via the imagePlane MEL command. Set the -showInAllViews flag to false and then set the looking through camera using the -lookThrough flag. For example, this command creates an image plane and sets the looking through camera to the side camera.
See imagePlane for more information regarding this command.
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