判断图片byte[]是否合法

bool IsValidPic(byte[] bts)
        {
            if (bts == null || bts.Length < 4)
            {
                return false;
            }
            string fileClass = "";
            int len = bts.Length;
            try
            {
                fileClass = bts[0].ToString();
                fileClass += bts[1].ToString();
                fileClass = fileClass.Trim();
                if (fileClass == "7173" || fileClass == "13780")//7173是gif;,13780是PNG;
                {
                    return true;
                }
                else      // JpgOrJpeg
                {
                    byte[] Jpg = new byte[4];

                    Jpg[0] = 0xff;

                    Jpg[1] = 0xd8;

                    Jpg[2] = 0xff;

                    Jpg[3] = 0xd9;

                    if (bts[0] == Jpg[0] && bts[1] == Jpg[1]
                        && bts[len - 2] == Jpg[2] && bts[len - 1] == Jpg[3])
                    {
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                }
            }
            catch (Exception ex)
            {
                //Log.WriteLog(e.ToString() + "  fileclass = " + fileClass);
                WriteLog.LogException(ex);
                return false;
            }
        }