//代码摘自网络,自己定义个类，将相关代码放进去即可使用
enum Dimensions 
        {
            Width,
            Height
        }
        enum AnchorPosition
        {
            Top,
            Center,
            Bottom,
            Left,
            Right
        }
static Image ScaleByPercent(Image imgPhoto, int Percent)
        {
            float nPercent = ((float)Percent/100);

            int sourceWidth = imgPhoto.Width;
            int sourceHeight = imgPhoto.Height;
            int sourceX = 0;
            int sourceY = 0;

            int destX = 0;
            int destY = 0; 
            int destWidth  = (int)(sourceWidth * nPercent);
            int destHeight = (int)(sourceHeight * nPercent);

            Bitmap bmPhoto = new Bitmap(destWidth, destHeight, PixelFormat.Format24bppRgb);
            bmPhoto.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);

            Graphics grPhoto = Graphics.FromImage(bmPhoto);
            grPhoto.InterpolationMode = InterpolationMode.HighQualityBicubic;

            grPhoto.DrawImage(imgPhoto, 
                new Rectangle(destX,destY,destWidth,destHeight),
                new Rectangle(sourceX,sourceY,sourceWidth,sourceHeight),
                GraphicsUnit.Pixel);

            grPhoto.Dispose();
            return bmPhoto;
        }
        static Image ConstrainProportions(Image imgPhoto, int Size, Dimensions Dimension)
        {
            int sourceWidth = imgPhoto.Width;
            int sourceHeight = imgPhoto.Height;
            int sourceX = 0;
            int sourceY = 0;
            int destX = 0;
            int destY = 0; 
            float nPercent = 0;

            switch(Dimension)
            {
                case Dimensions.Width:
                    nPercent = ((float)Size/(float)sourceWidth);
                    break;
                default:
                    nPercent = ((float)Size/(float)sourceHeight);
                    break;
            }
                
            int destWidth  = (int)(sourceWidth * nPercent);
            int destHeight = (int)(sourceHeight * nPercent);

            Bitmap bmPhoto = new Bitmap(destWidth, destHeight, PixelFormat.Format24bppRgb);
            bmPhoto.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);

            Graphics grPhoto = Graphics.FromImage(bmPhoto);
            grPhoto.InterpolationMode = InterpolationMode.HighQualityBicubic;

            grPhoto.DrawImage(imgPhoto, 
            new Rectangle(destX,destY,destWidth,destHeight),
            new Rectangle(sourceX,sourceY,sourceWidth,sourceHeight),
            GraphicsUnit.Pixel);

            grPhoto.Dispose();
            return bmPhoto;
        }

        static Image FixedSize(Image imgPhoto, int Width, int Height)
        {
            int sourceWidth = imgPhoto.Width;
            int sourceHeight = imgPhoto.Height;
            int sourceX = 0;
            int sourceY = 0;
            int destX = 0;
            int destY = 0; 

            float nPercent = 0;
            float nPercentW = 0;
            float nPercentH = 0;

            nPercentW = ((float)Width/(float)sourceWidth);
            nPercentH = ((float)Height/(float)sourceHeight);

            //if we have to pad the height pad both the top and the bottom
            //with the difference between the scaled height and the desired height
            if(nPercentH < nPercentW)
            {
                nPercent = nPercentH;