给图片加上水印

首先定义水印生成类：

    public class ImageWatermark
    {
        public static Image CreateWatermark(
           string fileName,
           string markStr,
           Font font,
           Color color,
           float opacity,
           ContentAlignment markAlign)
        {
            return CreateWatermark(
                Image.FromFile(fileName),
                markStr,
                font,
                color,
                opacity,
                markAlign);
        }

        public static Image CreateWatermark(
            Image image,
            string markStr,
            Font font,
            Color color,
            float opacity,
            ContentAlignment markAlign)
        {
            if (image == null)
            {
                throw new ArgumentNullException("iamge");
            }

            if (font == null)
            {
                font = new Font("宋体", 12F, FontStyle.Bold, GraphicsUnit.Point, 134);
            }

            if (string.IsNullOrEmpty(markStr))
            {
                return image;
            }

            Rectangle textRect = new Rectangle(Point.Empty, image.Size);

            StringFormat sf = new StringFormat();
            sf.Trimming = StringTrimming.EllipsisCharacter;

            switch (markAlign)
            {
                case ContentAlignment.TopLeft:
                    sf.LineAlignment = StringAlignment.Near;
                    sf.Alignment = StringAlignment.Near;
                    break;
                case ContentAlignment.TopCenter:
                    sf.LineAlignment = StringAlignment.Center;
                    sf.Alignment = StringAlignment.Near;
                    break;
                case ContentAlignment.TopRight:
                    sf.LineAlignment = StringAlignment.Far;
                    sf.Alignment = StringAlignment.Near;
                    break;
                case ContentAlignment.MiddleLeft:
                    sf.LineAlignment = StringAlignment.Near;
                    sf.Alignment = StringAlignment.Center;
                    break;
                case ContentAlignment.MiddleCenter:
                    sf.LineAlignment = StringAlignment.Center;
                    sf.Alignment = StringAlignment.Center;
                    break;
                case ContentAlignment.MiddleRight:
                    sf.LineAlignment = StringAlignment.Far;
                    sf.Alignment = StringAlignment.Center;
                    break;
                case ContentAlignment.BottomLeft:
                    sf.LineAlignment = StringAlignment.Near;
                    sf.Alignment = StringAlignment.Far;
                    break;
                case ContentAlignment.BottomCenter:
                    sf.LineAlignment = StringAlignment.Center;
                    sf.Alignment = StringAlignment.Far;
                    break;
                case ContentAlignment.BottomRight:
                    sf.LineAlignment = StringAlignment.Far;
                    sf.Alignment = StringAlignment.Far;
                    break;
            }

            Bitmap bmp = new Bitmap(image);
            color = Color.FromArgb((int)(255 * opacity), color);
            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.TextRenderingHint = TextRenderingHint.AntiAlias;

                using (SolidBrush brush = new SolidBrush(color))
                {
                    g.DrawString(
                        markStr,
                        font,
                        brush,
                        textRect,
                        sf);
                }
                g.Flush();
            }

            return bmp;
        }

        public static Image CreateWatermark(
            string fileName,
            string markFileName,
            Size markSize,
            float opacity,
            ContentAlignment markAlign)
        {
            return CreateWatermark(
                Image.FromFile(fileName),
                Image.FromFile(markFileName),
                markSize,
                opacity,
                markAlign);
        }

        public static Image CreateWatermark(
            Image image,
            Image markImage,
            Size markSize,
            float opacity,
            ContentAlignment markAlign)
        {
            if (image == null)
            {
                throw new ArgumentNullException("iamge");
            }

            if (markImage == null)
            {
                throw new ArgumentNullException("maskImage");
            }

            if (markSize == Size.Empty)
            {
                markSize = markImage.Size;
            }

            int width = image.Width;
            int height = image.Height;
            Rectangle maskRect = new Rectangle(Point.Empty, markSize);

            switch (markAlign)
            {
                case ContentAlignment.TopLeft:
                    maskRect.X = 2;
                    maskRect.Y = 2;
                    break;
                case ContentAlignment.TopCenter:
                    maskRect.X = (width - markSize.Width) / 2;
                    maskRect.Y = 2;
                    break;
                case ContentAlignment.TopRight:
                    maskRect.X = width - markSize.Width - 2;
                    maskRect.Y = 2;
                    break;
                case ContentAlignment.MiddleLeft:
                    maskRect.X = 2;
                    maskRect.Y = (height - markSize.Height) / 2;
                    break;
                case ContentAlignment.MiddleCenter:
                    maskRect.X = (width - markSize.Width) / 2;
                    maskRect.Y = (height - markSize.Height) / 2;
                    break;
                case ContentAlignment.MiddleRight:
                    maskRect.X = width - markSize.Width - 2;
                    maskRect.Y = (height - markSize.Height) / 2;
                    break;
                case ContentAlignment.BottomLeft:
                    maskRect.X = 2;
                    maskRect.Y = height - markSize.Height - 2;
                    break;
                case ContentAlignment.BottomCenter:
                    maskRect.X = (width - markSize.Width) / 2;
                    maskRect.Y = height - markSize.Height - 2;
                    break;
                case ContentAlignment.BottomRight:
                    maskRect.X = width - markSize.Width - 2;
                    maskRect.Y = height - markSize.Height - 2;
                    break;
            }

            Bitmap bmp = new Bitmap(image);

            using (ImageAttributes imageAttributes = new ImageAttributes())
            {
                ColorMap colorMap = new ColorMap();

                colorMap.OldColor = Color.FromArgb(255, 0, 255, 0);
                colorMap.NewColor = Color.FromArgb(0, 0, 0, 0);

                ColorMap[] remapTable = { colorMap };

                imageAttributes.SetRemapTable(remapTable, ColorAdjustType.Bitmap);

                float[][] colorMatrixElements = { 
                                                new float[] {1.0f,  0.0f,  0.0f,  0.0f, 0.0f},       
                                                new float[] {0.0f,  1.0f,  0.0f,  0.0f, 0.0f},        
                                                new float[] {0.0f,  0.0f,  1.0f,  0.0f, 0.0f},        
                                                new float[] {0.0f,  0.0f,  0.0f,  opacity, 0.0f},        
                                                new float[] {0.0f,  0.0f,  0.0f,  0.0f, 1.0f}};
                ColorMatrix wmColorMatrix = new ColorMatrix(colorMatrixElements);

                imageAttributes.SetColorMatrix(wmColorMatrix, ColorMatrixFlag.Default,
                    ColorAdjustType.Bitmap);

                using (Graphics g = Graphics.FromImage(bmp))
                {
                    g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                    using (Bitmap maskBmp = new Bitmap(markImage))
                    {
                        maskBmp.MakeTransparent();
                        g.DrawImage(
                            maskBmp,
                            maskRect,
                            0,
                            0,
                            markImage.Width,
                            markImage.Height,
                            GraphicsUnit.Pixel,
                            imageAttributes);
                    }
                    g.Flush();
                }
            }
            return bmp;
        }
    }

使用方法： ImageWatermark.CreateWatermark(this.fileName,this.picPath,Size.Empty,0.5F,ContentAlignment.BottomRight);