// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (SlidesDocument doc = Document.Load<SlidesDocument>(@"D:\test.pptx"))
{
TextWatermark watermark = new TextWatermark("Protected image", new Font("Arial", 8));
watermark.HorizontalAlignment = HorizontalAlignment.Center;
watermark.VerticalAlignment = VerticalAlignment.Center;
watermark.RotateAngle = 45;
watermark.SizingType = SizingType.ScaleToParentDimensions;
watermark.ScaleFactor = 1;
// Get all images from the first slide
WatermarkableImageCollection images = doc.Slides[0].FindImages();
// Add watermark to all found images
foreach (WatermarkableImage image in images)
{
image.AddWatermark(watermark);
}
doc.Save();
}