Add Watermark to Image in C#

add-diagonal-watermark-to-image.cs

// Load an existing PNG image
using (Image image = Image.Load("Image.png"))
{
    // Declare a String object with Watermark Text
    string theString = "45 Degree Rotated Text";

    // Create and initialize an instance of Graphics class and Initialize an object of SizeF to store image Size
    Graphics graphics = new Graphics(image);
    SizeF sz = graphics.Image.Size;

    // Creates an instance of Font, initialize it with Font Face, Size and Style
    Font font = new Font("Times New Roman", 20, FontStyle.Bold);

    // Create an instance of SolidBrush and set its various properties
    SolidBrush brush = new SolidBrush();
    brush.Color = Color.Red;
    brush.Opacity = 0;

    // Initialize an object of StringFormat class and set its various properties
    StringFormat format = new StringFormat();
    format.Alignment = StringAlignment.Center;
    format.FormatFlags = StringFormatFlags.MeasureTrailingSpaces;

    // Create an object of Matrix class for transformation
    Matrix matrix = new Matrix();

    // First a translation then a rotation                
    matrix.Translate(sz.Width / 2, sz.Height / 2);
    matrix.Rotate(-45.0f);

    // Set the Transformation through Matrix
    graphics.Transform = matrix;

    // Draw the string on Image Save output to disk
    graphics.DrawString(theString, font, brush, 0, 0, format);
    image.Save("watermarked.png");
}

add-watermark-to-image.cs

// Load an existing PNG image
using (Image image = Image.Load("image.png"))
{
    // Declare a String object with Watermark Text
    string theString = "This is watermark";

    // Create and initialize an instance of Graphics class and Initialize an object of SizeF to store image Size
    Graphics graphics = new Graphics(image);
    SizeF sz = graphics.Image.Size;

    // Creates an instance of Font, initialize it with Font Face, Size and Style
    Font font = new Font("Times New Roman", 20, FontStyle.Bold);

    // Create an instance of SolidBrush and set its various properties
    SolidBrush brush = new SolidBrush();
    brush.Color = Color.Red;
    brush.Opacity = 0;

    // Initialize an object of StringFormat class and set its various properties
    StringFormat format = new StringFormat();
    format.Alignment = StringAlignment.Center;
    format.FormatFlags = StringFormatFlags.MeasureTrailingSpaces;

    // Draw the string on Image Save output to disk
    graphics.DrawString(theString, font, brush, 0, 0, format);
    image.Save("watermarked.jpg");
}