更换工具栏的按钮图标，修改Toolbar按钮图标

 /// <summary>
        /// 加载Debug\Images目录下的图标或图片
        /// </summary>
        /// <param name="imgFileName">文件名,如：32_Print.png</param>
        /// <returns></returns>
        public static Bitmap LoadBitmap(string imgFileName)
        {
            try
            {
                string file = Application.StartupPath + @"\images\" + imgFileName;
                if (File.Exists(file))
                {
                    using (MemoryStream ms = new MemoryStream(File.ReadAllBytes(file)))
                    {
                        return new Bitmap(ms);
                    }
                }
                else
                    return null;
            }
            catch
            {
                return null;
            }
        }