核心代码

function checkBOM ($filename) { 
    global $auto; 
    $contents = file_get_contents($filename); 
    $charset[1] = substr($contents, 0, 1); 
    $charset[2] = substr($contents, 1, 1); 
    $charset[3] = substr($contents, 2, 1); 
    if (ord($charset[1]) == 239 && ord($charset[2]) == 187 && ord($charset[3]) == 191) { 
      if ($auto == 1) { 
       $rest = substr($contents, 3); 
       rewrite ($filename, $rest); 
       return ("<font color=red>BOM found, automatically removed.</font>"); 
      }else { 
       return ("<font color=red>BOM found.</font>"); 
      } 
    }else return ("BOM Not Found."); 
  }

php清除莫明输出,用于输出图片前把前面所有输出清空

只要在header前连用两个函数，就可以解决...

ob_get_clean();
ob_clean();
header("Content-type: image/jpeg");

附几个好用的BOM检测工具：
BOM检测.zip