Changes the contrast of the picture by the specified amount. Use the Contrast property to set the absolute contrast for the picture.
expression.IncrementContrast(Increment)
expression Required. An expression that returns a PictureFormat object.
Increment Required Single. Specifies how much to change the value of the Contrast property for the picture. A positive value increases the contrast; a negative value decreases the contrast.
Remarks
You cannot adjust the contrast of a picture past the upper or lower limit for the Contrast property. For example, if the Contrast property is initially set to 0.9 and you specify 0.3 for the Increment argument, the resulting contrast level will be 1.0, which is the upper limit for the Contrast property, instead of 1.2.
Example
This example increases the contrast for all pictures on myDocument that aren't already set to maximum contrast.
Set myDocument = ActivePresentation.Slides(1)
For Each s In myDocument.Shapes
If s.Type = msoPicture Then
s.PictureFormat.IncrementContrast 0.1
End If
Next