i've seen some downvotes , here is a small example of using curl to upload image .

<?php
$target="http://youraddress.tld/example/upload.php";

# http://php.net/manual/en/curlfile.construct.php

// Create a CURLFile object / procedural method 
$cfile = curl_file_create('resource/test.png','image/png','testpic'); // try adding 

// Create a CURLFile object / oop method 
#$cfile = new CURLFile('resource/test.png','image/png','testpic'); // uncomment and use if the upper procedural method is not working.

// Assign POST data
$imgdata = array('myimage' => $cfile);

$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $target);
curl_setopt($curl, CURLOPT_USERAGENT,'Opera/9.80 (Windows NT 6.2; Win64; x64) Presto/2.12.388 Version/12.15');
curl_setopt($curl, CURLOPT_HTTPHEADER,array('User-Agent: Opera/9.80 (Windows NT 6.2; Win64; x64) Presto/2.12.388 Version/12.15','Referer: http://someaddress.tld','Content-Type: multipart/form-data'));
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false); // stop verifying certificate
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true); 
curl_setopt($curl, CURLOPT_POST, true); // enable posting
curl_setopt($curl, CURLOPT_POSTFIELDS, $imgdata); // post images 
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true); // if any redirection after upload
$r = curl_exec($curl); 
curl_close($curl);

?>

There are "@" issue on multipart POST requests.

Solution for PHP 5.5 or later:
- Enable CURLOPT_SAFE_UPLOAD.
- Use CURLFile instead of "@".

Solution for PHP 5.4 or earlier:
- Build up multipart content body by youself.
- Change "Content-Type" header by yourself.

The following snippet will help you :D

<?php

/**
 * For safe multipart POST request for PHP5.3 ~ PHP 5.4.
 * 
 * @param resource $ch cURL resource
 * @param array $assoc "name => value"
 * @param array $files "name => path"
 * @return bool
 */
function curl_custom_postfields($ch, array $assoc = array(), array $files = array()) {
    
    // invalid characters for "name" and "filename"
    static $disallow = array("\0", "\"", "\r", "\n");
    
    // build normal parameters
    foreach ($assoc as $k => $v) {
        $k = str_replace($disallow, "_", $k);
        $body[] = implode("\r\n", array(
            "Content-Disposition: form-data; name=\"{$k}\"",
            "",
            filter_var($v), 
        ));
    }
    
    // build file parameters
    foreach ($files as $k => $v) {
        switch (true) {
            case false === $v = realpath(filter_var($v)):
            case !is_file($v):
            case !is_readable($v):
                continue; // or return false, throw new InvalidArgumentException
        }
        $data = file_get_contents($v);
        $v = call_user_func("end", explode(DIRECTORY_SEPARATOR, $v));
        $k = str_replace($disallow, "_", $k);
        $v = str_replace($disallow, "_", $v);
        $body[] = implode("\r\n", array(
            "Content-Disposition: form-data; name=\"{$k}\"; filename=\"{$v}\"",
            "Content-Type: application/octet-stream",
            "",
            $data, 
        ));
    }
    
    // generate safe boundary 
    do {
        $boundary = "---------------------" . md5(mt_rand() . microtime());
    } while (preg_grep("/{$boundary}/", $body));
    
    // add boundary for each parameters
    array_walk($body, function (&$part) use ($boundary) {
        $part = "--{$boundary}\r\n{$part}";
    });
    
    // add final boundary
    $body[] = "--{$boundary}--";
    $body[] = "";
    
    // set options
    return @curl_setopt_array($ch, array(
        CURLOPT_POST       => true,
        CURLOPT_POSTFIELDS => implode("\r\n", $body),
        CURLOPT_HTTPHEADER => array(
            "Expect: 100-continue",
            "Content-Type: multipart/form-data; boundary={$boundary}", // change Content-Type
        ),
    ));
}

?>

Simple function to construct CURLFile from the file path:

function makeCurlFile($file){
$mime = mime_content_type($file);
$info = pathinfo($file);
$name = $info['basename'];
$output = new CURLFile($file, $mime, $name);
return $output;
}

Then construct an array of all the form fields you want to post. For each file upload just add the CURLFiles.

$ch = curl_init("https://api.example.com");
$mp3 =makeCurlFile($audio);
$photo = makeCurlFile($picture);
$data = array('mp3' => $mp3, 'picture' => $photo, 'name' => 'My latest single', 'description' => 'Check out my newest song');
curl_setopt($ch, CURLOPT_POST,1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
$result = curl_exec($ch);
if (curl_errno($ch)) {
   $result = curl_error($ch);
}
curl_close ($ch);

CURLFile() is great with Content-Type: "multipart/form-data".

But, if you want to upload just a single file (e.g. for the Wordpress media API), then you have to do this WITHOUT using CURLFile().  i.e.

1. Set the relevant CURLOPT_HTTPHEADER values:
"Content-Type: image/jpeg"  
"Content-Disposition: attachment; filename=myfile.jpg"
(where the content-type needs to be the appropriate mime-type for the uploaded file, and filename is the basename of the file).

2. Post the file contents directly within CURLOPT_POSTFIELDS:
file_get_contents($filename);

This is how to upload two or more files at once with cURLFile.

With modern browser support, you can upload many files at one time if the multiple attribute is given.

<input type="file" name="file[]" muliple />

With outdated browser you can place many input elements to do that.

<input type="file" name="file[]" />
<input type="file" name="file[]" />
<input type="file" name="file[]" />

Here's the php code to handle multiple uploads.

<?php
    $cURLHandle = curl_init();
    // ... more code ...
    $postFields = array(
        'file[0]' => new cURLFile($file1, $mimetype1, $basename1),
        'file[1]' => new cURLFile($file2, $mimetype2, $basename2)
    )
    
    curl_setopt($cURLHandle, CURLOPT_POSTFIELDS, $fields);
    
    // ... more code ...
?>
Each file's mimetype can be determined by finfo, and its basename can be fetched from $_FILES if it is uploaded from the client-end or by pathinfo() if it is locally stored.

Don't leave the square brackets in ''file[0]'' empty like 'file[]', in that way only the last file will be received by the remote server.