程序代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Drawing;

namespace myTools
{
    public static partial class bk
    {
        //------------------------------------------------------------
        // 功能：将图像转换为指定尺寸的图像，并在目标区域内居中
        // 参数：sourceImageFileName -- 源文件名
        //       targetImageFileName -- 目标文件名
        //                     width -- 目标区域像素宽度
        //                    height -- 目标区域像素高度
        // 备注：转换图像为96DPI、白色背景、24位色的PNG格式
        //------------------------------------------------------------
        public static Boolean Convert_Image(String sourceImageFileName, String targetImageFileName, Int32 width, Int32 height)
        {
            Boolean success = false;

            if (sourceImageFileName.ToUpper().EndsWith(".JPG") || sourceImageFileName.ToUpper().EndsWith(".PNG"))
            {
                Int32 top = 0;      // 新图像在目标区域中的顶坐标
                Int32 left = 0;     // 新图像在目标区域中的左坐标

                Image sourceImage = Image.FromFile(sourceImageFileName);
                Bitmap newBitmap = new Bitmap(width, height, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
                newBitmap.SetResolution(96, 96);
                if (sourceImage.Width > sourceImage.Height)
                {
                    height = sourceImage.Height * width / sourceImage.Width;
                    top = (newBitmap.Height - height) / 2;
                }
                else
                {
                    width = sourceImage.Width * height / sourceImage.Height;
                    left = (newBitmap.Width - width) / 2;
                }
                Rectangle rect = new Rectangle(left, top, width, height);

                Graphics theGraphics = Graphics.FromImage(newBitmap);
                theGraphics.Clear(System.Drawing.Color.White);
                theGraphics.DrawImage(sourceImage, rect);
                newBitmap.Save(targetImageFileName, System.Drawing.Imaging.ImageFormat.Png);

                sourceImage.Dispose();
                newBitmap.Dispose();
                theGraphics.Dispose();

                success = true;
            }

            return success;
        }
    }
}