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' 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")
    Using watermark = New ImageWatermark("D:\logo.png")
        Dim effects As New SlidesImageEffects()
        effects.Brightness = 0.7
        effects.Contrast = 0.6
        effects.ChromaKey = Color.Red
        effects.BorderLineFormat.Enabled = True
        effects.BorderLineFormat.Weight = 1
        doc.AddImageWatermark(watermark, effects)
    End Using
    doc.Save()
End Using