产生随机图片,并返回图片内的文字

/**//// <summary>
        /// 产生随机图片,并返回图片内的文字
        /// </summary>
        /// <param name="img">要显示图片的控件</param>        
        /// <param name="path">图片显示的文字</param>
        public string R_random(System.Web.UI.WebControls.Image img,string path)
        {                
            string tmp;//文字
            Random rnd=new Random();
            tmp=rnd.Next().ToString().Substring(0,4);
            Graphics g=null;
            Bitmap bmp=new Bitmap(50,100);
            g=Graphics.FromImage(bmp);
            SizeF rec=g.MeasureString(tmp,new Font("宋体",12));
            int nwidth=(int)rec.Width;
            int nheight=(int)rec.Height;
            g.Dispose();
            bmp.Dispose();
            bmp=new Bitmap(nwidth,nheight);
            g=Graphics.FromImage(bmp);
            g.FillRectangle(new SolidBrush(Color.FromArgb(90, 126, 220)), new Rectangle(0,0,nwidth,nheight));
            g.DrawString(tmp,new Font("宋体",12), new SolidBrush(Color.Yellow), new PointF(0, 0));
            string fff=path+ @"tmp\validation.gif";
            bmp.Save(fff,System.Drawing.Imaging.ImageFormat.Gif);                
            img.ImageUrl=fff;
            return tmp;