' For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
Using doc As SlidesDocument = Document.Load(Of SlidesDocument)("D:\test.pptx")
Dim watermark As 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
Dim images As WatermarkableImageCollection = doc.Slides(0).FindImages()
' Add watermark to all found images
For Each image As WatermarkableImage In images
image.AddWatermark(watermark)
Next
doc.Save()
End Using