Your image is not Resized to the specified width and height because it will maintain aspect ratio. When the image is drawn on top it will make the 'unknown' part black. Settings the background to Transparent and using the Extent method should solve your problem.
using (MagickImage image = new MagickImage("example.jpg")) { using (MagickImage watermark = new MagickImage("watermark.png")) { //resize watermark to fit the proof image watermark.Resize(image.Width, image.Height); watermark.BackgroundColor = MagickColor.Transparent; watermark.Extent(image.Width, image.Height); image.Composite(watermark, Gravity.Center, CompositeOperator.Dissolve, "11"); image.Write("examplewatermarked.png"); } }