While working on one of my friend’s blog, I got a chance to write a code snippet which helps to add water mark for all images automatically. And I created a simple library do this. I added few extra options, it may not work properly, I didn’t tested it

namespace dotnetthoughts.net
{
    using System.Drawing;

    /// <summary>
    /// </summary>
    public class WaterMarker
    {
        /// <summary>
        /// Sets / Gets color of the Watermark text.
        /// </summary>
        public Color Color { get; set; }

        /// <summary>
        /// Sets or Gets Font of Watermark Text.
        /// </summary>
        public Font Font { get; set; }

        /// <summary>
        /// Sets or Gets Alpha value of the Color.
        /// </summary>
        /// <remarks>
        /// It should be between 0 and 255.
        /// </remarks>
        public int Alpha { get; set; }

        /// <summary>
        /// Sets or Gets the Watermark Text
        /// </summary>
        public string WatermarkText { get; set; }

        /// <summary>
        /// Sets or Gets the alignment of Watermark text
        /// </summary>
        public ContentAlignment TextAlignment { get; set; }

        /// <summary>
        /// Creates an instance of Watermarker class.
        /// </summary>
        public WaterMarker()
        {
            Color = Color.White;
            Alpha = 200;
            Font = new Font(FontFamily.GenericSerif, 20);
            WatermarkText = "dotnetthoughts.net";
            TextAlignment = ContentAlignment.BottomCenter;
        }

        /// <summary>
        /// Adds the watermark text to the given image.
        /// </summary>
        /// <param name="image">Image to apply watermark</param>
        /// <param name="waterMarkText">Watermark Text</param>
        /// <returns>Image</returns>
        public Image Apply(Image image, string waterMarkText)
        {
            WatermarkText = waterMarkText;
            using (Graphics graphics = Graphics.FromImage(image))
            {
                var size = 
                    graphics.MeasureString(WatermarkText, Font);
                var brush = 
                    new SolidBrush(Color.FromArgb(Alpha, Color));
                graphics.DrawString
                    (WatermarkText, Font, brush, 
                    GetTextPosition(image, size));
            }
            return image;
        }

        /// <summary>
        /// Adds the watermark text to the given image.
        /// </summary>
        /// <param name="image">Image to apply watermark</param>
        /// <returns>Image</returns>
        public Image Apply(Image image)
        {
            return Apply(image, WatermarkText);
        }
        
        private PointF GetTextPosition(Image image, SizeF size)
        {
            PointF point = default(PointF);
            switch (TextAlignment)
            {
                case ContentAlignment.BottomCenter:
                    point = new PointF((image.Width - size.Width) / 2, 
                        (image.Height - size.Height));
                    break;
                case ContentAlignment.BottomLeft:
                    point = new PointF(0, (image.Height - size.Height));
                    break;
                case ContentAlignment.BottomRight:
                    point = new PointF((image.Width - size.Width),
                        (image.Height - size.Height));
                    break;
                case ContentAlignment.MiddleCenter:
                    point = new PointF((image.Width - size.Width) / 2,
                        (image.Height - size.Height) / 2);
                    break;
                case ContentAlignment.MiddleLeft:
                    point = new PointF(0, (image.Height - size.Height) / 2);
                    break;
                case ContentAlignment.MiddleRight:
                    point = new PointF((image.Width - size.Width), 
                        (image.Height - size.Height) / 2);
                    break;
                case ContentAlignment.TopCenter:
                    point = new PointF((image.Width - size.Width) / 2, 0);
                    break;
                case ContentAlignment.TopLeft:
                    point = new PointF(0, 0);
                    break;
                case ContentAlignment.TopRight:
                    point = new PointF((image.Width - size.Width), 0);
                    break;
            }
            return point;
        }
    }
}

Happy Coding