GroupDocs.Watermar API supports adding the following image file types as image watermark:
Bmp; Png; Gif; Jpeg. Add image watermark from local file Following code snippet shows how to add ImageWatermark to a document. If the document consists of multiple parts (pages, worksheets, slides, frames etc), the watermark will be added to all of them.
advanced_usage.adding_image_watermarks.AddImageWatermark
// Specify an absolute or relative path to your document. Ex: "C:\\Docs\\presentation.Pptx" Watermarker watermarker = new Watermarker("presentation....pptx" Watermarker watermarker = new...Watermarker ( "presentation.pptx" ); // Use path to the image...