Brightness Property
Returns or sets the brightness of the specified picture or OLE object. The value for this property must be a number from 0.0 (dimmest) to 1.0 (brightest). Read/write Single.
Example
This example sets the brightness for shape one on myDocument. Shape one must be either a picture or an OLE object.
Set myDocument = ActivePresentation.Slides(1)
myDocument.Shapes(1).PictureFormat.Brightness = 0.3