The Transparent property returns True if the bitmap is transparent.
Examples:
The following VBA example displays the number of transparent bitmaps that are on the active page.
Sub Test()
Dim s As Shape, n As Long
n = 0
For Each s In ActivePage.Shapes
If s.Type = cdrBitmapShape Then
If s.Bitmap.Transparent Then n = n + 1
End If
Next s
MsgBox "There are " & n & " transparent bitmaps"
End Sub