Unable to add an Image to a powerpoint presentation using open xml - image

Question

I am using the following code to add a new slide to a ppt file and add an image. I am using Open XML 2.5 SDK.
A new slide is getting added but not the image. Is there anything wrong in this code?
int position = 1;
using (PresentationDocument presentationDocument = PresentationDocument.Open("c.pptx", true))
{
PresentationPart presentationPart = presentationDocument.PresentationPart;
Slide slide = new Slide(new CommonSlideData(new ShapeTree()));
NonVisualGroupShapeProperties nonVisualProperties = slide.CommonSlideData.ShapeTree.AppendChild(new NonVisualGroupShapeProperties());
nonVisualProperties.NonVisualDrawingProperties = new NonVisualDrawingProperties() { Id = 1, Name = "" };
nonVisualProperties.NonVisualGroupShapeDrawingProperties = new NonVisualGroupShapeDrawingProperties();
nonVisualProperties.ApplicationNonVisualDrawingProperties = new ApplicationNonVisualDrawingProperties();
// Specify the group shape properties of the new slide.
slide.CommonSlideData.ShapeTree.AppendChild(new GroupShapeProperties());
// Create the slide part for the new slide.
SlidePart slidePart = presentationPart.AddNewPart<SlidePart>();
// Save the new slide part.
slide.Save(slidePart);
string imgId = "rId" + new Random().Next(2000).ToString();
ImagePart imagePart = slidePart.AddImagePart(ImagePartType.Png, imgId);
using (FileStream stream = new FileStream("a.png", FileMode.Open))
{
stream.Position = 0;
imagePart.FeedData(stream);
}
slide.Save(slidePart);
// Modify the slide ID list in the presentation part.
// The slide ID list should not be null.
SlideIdList slideIdList = presentationPart.Presentation.SlideIdList;
// Find the highest slide ID in the current list.
uint maxSlideId = 1;
SlideId prevSlideId = null;
foreach (SlideId slideId in slideIdList.ChildElements)
{
if (slideId.Id > maxSlideId)
{
maxSlideId = slideId.Id;
}
position--;
if (position == 0)
{
prevSlideId = slideId;
}
}
maxSlideId++;
// Get the ID of the previous slide.
SlidePart lastSlidePart;
if (prevSlideId != null)
{
lastSlidePart = (SlidePart)presentationPart.GetPartById(prevSlideId.RelationshipId);
}
else
{
lastSlidePart = (SlidePart)presentationPart.GetPartById(((SlideId)(slideIdList.ChildElements[0])).RelationshipId);
}
// Use the same slide layout as that of the previous slide.
if (null != lastSlidePart.SlideLayoutPart)
{
slidePart.AddPart(lastSlidePart.SlideLayoutPart);
}
// Insert the new slide into the slide list after the previous slide.
SlideId newSlideId = slideIdList.InsertAfter(new SlideId(), prevSlideId);
newSlideId.Id = maxSlideId;
newSlideId.RelationshipId = presentationPart.GetIdOfPart(slidePart);
// Save the modified prsentation.
presentationPart.Presentation.Save();
Thanks in advance.