expression.TransparencyColor
expression Required. An expression that returns one of the objects in the Applies To list.
Remarks
If you want to be able to see through the transparent parts of the picture all the way to the objects behind the picture, you must set the Visible property of the picture's FillFormat object to False. If your picture has a transparent color and the Visible property of the picture's FillFormat object is set to True, the picture's fill will be visible through the transparent color, but objects behind the picture will be obscured.
Example
This example sets the color that has the RGB value returned by the function RGB(0, 0, 255) as the transparent color for shape one on myDocument. For the example to work, shape one must be a bitmap.
blueScreen = RGB(0, 0, 255)
Set myDocument = Worksheets(1)
With myDocument.Shapes(1)
With .PictureFormat
.TransparentBackground = True
.TransparencyColor = blueScreen
End With
.Fill.Visible = False
End With