问人脸检测返回错误码-1102（SDK_IMAGE_DECODE_FAILED）怎么回事？

<?php

$appid = "*************";

$bucket = "tencentyun";

$secret_id = "*****************************";

$secret_key = "****************************";

$expired = time() + 2592000;

$current = time();

$rdm = rand();

$srcStr = 'a='.$appid.'&b='.$bucket.'&k='.$secret_id.'&e='.$expired.'&t='.$current.'&r='.$rdm.'&f=';

$signStr = base64_encode(hash_hmac('SHA1', $srcStr, $secret_key, true).$srcStr);

$post_data = array(

'appid' => $appid,

'mode' => 0,//全部人脸

'image' => "c.jpg"//正常的一张照片，跟PHP文件放在同一个目录下

);

$ch=curl_init();

curl_setopt($ch,CURLOPT_URL,"http://recognition.image.myqcloud.com/face/detect");

curl_setopt($ch, CURLOPT_HTTPHEADER, array('host: recognition.image.myqcloud.com','content-type: multipart/form-data','Authorization: '.$signStr));

curl_setopt($ch,CURLOPT_SSL_VERIFYPEER,FALSE);

curl_setopt($ch,CURLOPT_SSL_VERIFYHOST,FALSE);

curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);

curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);

$output=curl_exec($ch);

curl_close($ch);

$jsoninfo=json_decode($output,true);

print_r($jsoninfo);

运行输出：

[root@VM_97_138_centos face]# php txface.php

Array

(

[code] => -1102

[message] => SDK_IMAGE_DECODE_FAILED

[data] => Array

(

[session_id] =>

[face] => Array

(

)

)

)

[root@VM_97_138_centos face]#