帮忙看下小弟我这样写的扩展方法为什么点不出来

帮忙看下我这样写的扩展方法为什么点不出来？
using System;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.IO;


namespace System
{
    public static class ImageExtensions
    {
        public static Image GetThumbImageByWidth(this Image original, int nwidth)
        {
            if (original.Width < nwidth)
                return original;
            else
            {
                var width = 0; var height = 0;
                float scale = (float)nwidth / original.Width;

                width = nwidth;
                height = (int)Math.Round(original.Height * scale);

                var bitmap = new Bitmap(width, height);
                var g = Graphics.FromImage(bitmap);
                g.DrawImage(original, 0, 0, width, height);
                g.SmoothingMode = SmoothingMode.HighQuality;

                return bitmap;
            }
        }
    }
} 扩展方法

[解决办法]
对象实例才能点