Below is the code example, which illustrates, how to add an image in PPTX presentation. I have also attached the source presentation, image and output presentation with this post.
C#
//Read the source pptx
PresentationEx pres = new PresentationEx(@"c:\source\source.pptx");
//Access the first slide
SlideEx sld = pres.Slides[0];
//Read the image
Image img = Bitmap.FromFile(@"c:\BlueHills.jpg");
//Add the image in Presentation.Images collection
ImageEx imgEx=pres.Images.AddImage(img);
//Add the pictureframe
sld.Shapes.AddPictureFrame(ShapeTypeEx.Rectangle, 120, 110, 480, 320, imgEx);
//Write the presentation on disk
pres.Write("c:\\source\\output.pptx");