Example of use watermarks:

string output = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");

IConversion conversion = await FFmpeg.Conversions.FromSnippet.SetWatermark(Resources.Mp4WithAudio, output, Resources.PngSample, Position.Center);
IConversionResult result = await conversion.Start();

Watermark can be set in different position in a video:

/// <summary>
///     Melt watermark into video
/// </summary>
/// <param name="inputPath">Input video path</param>
/// <param name="outputPath">Output file</param>
/// <param name="inputImage">Watermark</param>
/// <param name="position">Position of watermark</param>
/// <returns>Conversion result</returns>
Task<IConversion> SetWatermark(string inputPath, string outputPath, string inputImage, Position position);
Big Buck Bunny with melted watermark

Big Buck Bunny with melted watermark in the center