Update Background Image

 

Views menu > Update Background Image (available only when a viewport background is displayed)

This command updates the background image displayed in the active viewport. If the active viewport is not displaying a background image, this command is unavailable.

Use this command to update the background for changes that are not updated automatically, such as the following:

• Reassigning the map, or changing any parameters affecting the map in the Materials Editor, the Environment dialog, or the Viewport Background dialog.
• Changing the rendering resolution and aspect ratio. The following changes update the viewport background image automatically:
• Changing the camera view.
• Undo (for views).
• Undo (for objects).
• Assigning a different view type.
• Toggling Safe Frames display on or off.
• Changing the rendering parameters.
• Moving the time slider when the viewport contains an animated background image.

Note : Viewports can use the current Environment Map (set on the Environment panel of the Environment and Effects dialog) as the background image.

Procedures

To update the background image displayed in a viewport:

1. Activate a viewport that contains a background image.
2. Choose Views menu > Update Background Image.

source : 3ds Max Help