steps.txt

'import_http_files' => array(
  'robot' => '/http/import',
  'url' => array(
    'http://cdn-ak.f.st-hatena.com/images/fotolife/m/mgng/20130130/20130130125239.png',
    'http://cdn-ak.f.st-hatena.com/images/fotolife/m/mgng/20130130/20130130105748.png',
  ),
),

↓

'import_http_files' => array(
  'robot' => '/html/convert',
  'url' => 'http://google.com',
),

transloadit_test.php

<?php
/**
 * transloadit API テスト
 */

// via: https://transloadit.com/accounts/credentials
$auth_key = 'メモったAuth Key';
$auth_sec = 'メモったAuth Secret';;

// params と signature を作成
$params = json_encode( array(
  // 認証用
  'auth' => array(
    'expires' => gmdate( 'Y/m/d H:i:s+00:00', strtotime( '+1 hour' ) ),
    'key' => $auth_key,
  ),
  // 処理内容
  'steps' => array(
    // web上のファイル使いますよ宣言
    'import_http_files' => array(
      'robot' => '/http/import',
      'url' => array(
        'http://cdn-ak.f.st-hatena.com/images/fotolife/m/mgng/20130130/20130130125239.png',
        'http://cdn-ak.f.st-hatena.com/images/fotolife/m/mgng/20130130/20130130105748.png',
      ),
    ),
    // import_http_files に指定した画像を 100*100 にリサイズしますよ宣言
    'resize_images' => array(
      'use' => 'import_http_files',
      'robot' => '/image/resize',
      'format' => 'jpg',
      'width' => 100,
      'height' => 100,
    ),
  )
));
$signature = hash_hmac( 'sha1', $params, $auth_sec );

// API に params と signature を POST して認証、assembly_url を取得
$auth = json_decode( post( 'http://api2.transloadit.com/assemblies', array(
  'params' => $params,
  'signature' => $signature,
) ) );
$assembly_url = $auth->assembly_url;

// sleep(1) しつつ assembly_url にリクエスト投げて処理状況確認
// ASSEMBLY_COMPLETED になってたら URL 表示
for( $i=0; $i<5; $i++ ) {
  sleep(1);
  $res = json_decode( file_get_contents( $assembly_url ) );
  if ( $res->ok === 'ASSEMBLY_COMPLETED' ) {
    foreach( $res->results->resize_images as $image ) {
      echo $image->url, "\n";
    }
    break;
  }
}

/**
 * 簡易POST関数
 * @param string $url
 * @param array $data OPTIONAL
 * @return string
 */
function post ( $url, $data = array() ) {
  return file_get_contents( $url, false, stream_context_create(
    array(
      'http'=>array(
        'method' => 'POST',
        'header' => "Content-type: application/x-www-form-urlencoded\r\n",
        'content'=> http_build_query( $data ),
      )
    )
  ));
}