如何在Wordpress中将GIF图像转换为静态JPG图像

Question

所以我有一个表格来上传GIF图像，我想保留动画GIF图像并转换/创建一个新的静态图像从GIF（获得第一帧）到JPG。

这是我的表格：

<form action="" id="primaryPostForm" method="POST" enctype="multipart/form-data">

        <fieldset>
    <p>Upload funny pictures, paste pictures or YouTube URL, accepting GIF/JPG/PNG (Max size: 3MB)</p>
            <label for="postTitle"><?php _e('* Title of the image:', 'framework') ?></label>

            <input type="text" name="postTitle" id="postTitle" class="required" value="<?php if ( isset( $_POST['postTitle'] ) ) echo $_POST['postTitle']; ?>" />
        </fieldset>

    <div id="file-input" class="file-field">
    <input type="file" name="thumbnail" id="thumbnail">
    </div>

这是我的验证码：

// validation
wp_register_script( 'validation', 'http://ajax.aspnetcdn.com/ajax/jquery.validate/1.9/jquery.validate.min.js', array( 'jquery' ) );
wp_enqueue_script( 'validation' );
if ( isset( $_POST['submitted'] ) && isset( $_POST['post_nonce_field'] ) && wp_verify_nonce( $_POST['post_nonce_field'], 'post_nonce' ) ) {
    if ( trim( $_POST['postTitle'] ) === '' ) {
        $postTitleError = 'Please enter a title.';
        $hasError = true;
    }
    $post_information = array(
        'post_title' => wp_strip_all_tags( $_POST['postTitle'] ),
        'post_content' => $_POST['postContent'],
        'post_type' => 'post',
        'post_status' => 'pending'
    );
$new_post = wp_insert_post( $post_information );
if (!function_exists('wp_generate_attachment_metadata')){
                require_once(ABSPATH . "wp-admin" . '/includes/image.php');
                require_once(ABSPATH . "wp-admin" . '/includes/file.php');
                require_once(ABSPATH . "wp-admin" . '/includes/media.php');
            }
             if ($_FILES) {
                foreach ($_FILES as $file => $array) {
                    if ($_FILES[$file]['error'] !== UPLOAD_ERR_OK) {
                        return "upload error : " . $_FILES[$file]['error'];
                    }
                    $attach_id = media_handle_upload( $file, $new_post );
                }   
            }
            if ($attach_id > 0){
                //and if you want to set that image as Post  then use:
                update_post_meta($new_post,'_thumbnail_id',$attach_id);
            }
}

我怎样才能做到这一点？