By default, the Visible property is True, so any picture on the layer's background is visible. The Visible property specifies whether the layer's background is visible or hidden. The Picture / ExtraPicture property adds a picture on the layer's background. Use the Value property to specify a solid / EBN color to be applied on the layer's background. The Selectable property specifies whether the layer's background is selectable. 

The Visible property of the Background object, affects all the pictures / colors being shown on the layer's background. In order to prevent showing portions of the layer you can use any of the following properties:

• Visible property of the LPicture object, specifies if the picture is shown or hidden on the layer's background.
• Visible property of the LColor object, specifies if the color is visible or hidden.