From Microsoft Community - you can use Shape.AutoShapeType to "crop to shape."
Sub CropToCircle()
Dim shp As Shape
Set shp = ActivePresentation.Slides(1).Shapes(1)
If shp.Type = msoLinkedPicture Or shp.Type = msoPicture Then
shp.AutoShapeType = msoShapeOval
End If
End Sub