Applying gradient to background image

I want to import the background as an image from the master slide in the attached ppt.
I have been able to extract the groupshape as a png image, but I want to apply the Gradient to that bg image as well.
Can someone please tell me how to do that?
Here’s my code:
public System.Drawing.Bitmap GroupShapeToBitMap(Shape sh)
{
GroupShape groupShape = (GroupShape)sh;

System.Drawing.Bitmap groupImage = (System.Drawing.Bitmap)sh.Parent.GetThumbnail(
new Object[] { sh }, 1, 1, sh.ShapeRectangle);

groupImage.MakeTransparent(System.Drawing.Color.White); // TBD if white should be kept as the transparent color

return groupImage;
}


You can use a workaround mentioned by me here.