Open the Select Background Image Window by selecting "Set Background Image" item from the "Model" menu.
The Select Background Image Window is composed of the direction frame, and the None and File... buttons. Use the direction tab to select which direction you want to chose a background for, and the File... button to select the image. To unassign a background image, select the view direction and press the None button.
To reposition or resize the background image in the viewport, use the Move Background Image tool or the Scale Background Image tool.
Press Ok to keep your changes or press Cancel to ignore any changes.