Attach image to post in Wordpress XMLRPC

Yes it is possible to do it, if Wordpress version is 3.5 or greater,when using the code for uploading file/image you can set the post_id.The flow I used for new posts with featured images is like this:

1. use the newPost function and post the content without the featuredimage and also set publish to false, record the post_id returned bythis

2. upload the image and set the post_id to the id of the post justposted, record the image_id

3. when done edit the post and set the wp_post_thumbnail equal to theimage_id you just uploaded and also set publish to true(if needed)

Important:The mime type is important, it must be "image/jpg" or "image/png" please see documentation, if mime type is worng like "jpg" attaching will fail.

Tip:For debugging, if you get a generic error from wordpress and you can't figure out why you can check the wordpress code and even edit it, adding debugging/tracing calls and hopefully you can figure out the cause.

This is an example of a post with category, image and tags. It requires class-IXR.php
https://github.com/WordPress/WordPress/blob/master/wp-includes/class-IXR.php
and mime_content_type function
https://github.com/caiofior/storebaby/blob/master/magmi/plugins/extra/general/socialnotify/wp/mimetype.php

        $client = new IXR_Client($url);        $content = array(            'post_status' => 'draft',            'post_type' => 'post',            'post_title' => 'Title',            'post_content' => 'Message',             // categories ids            'terms' => array('category' => array(3))        );        $params = array(0, $username, $password, $content);        $client->query('wp.newPost', $params);        $post_id = $client->getResponse();        $content = array(            'name' => basename('/var/www/sb/img.jpeg'),            'type' => mime_content_type('/var/www/sb/img.jpeg'),            'bits' => new IXR_Base64(file_get_contents('/var/www/sb/img.jpeg')),            true        );        $client->query('metaWeblog.newMediaObject', 1, $username, $password, $content);        $media = $client->getResponse();        $content = array(            'post_status' => 'publish',            // Tags            'mt_keywords' => 'tag1, tag2, tag3',            'wp_post_thumbnail' => $media['id']        );        $client->query('metaWeblog.editPost', $post_id, $username, $password, $content, true);

My version if you want to use only wp.newPost and wp.editPost

include_once( ABSPATH . WPINC . '/class-IXR.php' );include_once( ABSPATH . WPINC . '/class-wp-http-ixr-client.php' );    $usr = 'username_on_the_server_side';    $pwd = 'password_on_the_server_side';    $xmlrpc = 'server side xmlrpc.php url';    $client = new IXR_Client($xmlrpc);    ////////////  IMAGE UPLOAD AND ATTACHMENT POST CREATION  ///////////       $img_attach = 'link to the image';       $img_attach_content = array(                'name' => basename($img_attach),                'type' => mime_content_type($img_attach),                'bits' => new IXR_Base64(file_get_contents($img_attach)),                        );        $status = $client->query( 'wp.uploadFile','1',  $usr, $pwd, $img_attach_content );        $image_returnInfo = $client ->getResponse();   ////////////  POST CREATION  ///////////        $custom_fields = array(                           array( 'key' => 'blabla1', 'value' => 'blabla1_value' ),                          array( 'key' => 'blabla12', 'value' => 'blabla1_value2')                          );         $post_content = array(            'post_type' => 'post',            'post_status' => 'draft', //for now            'post_title' => 'XMLRPC Test',            'post_author' => 3,            'post_name' => 'XMLRPC Test',            'post_content' => 'XMLRPC Test Content',            'custom_fields' => $custom_fields        );    $res = $client -> query('wp.newPost',1, $usr, $pwd, $post_content);    $postID =  $client->getResponse();    if(!$res)        echo 'Something went wrong....';    else {            echo 'The Project Created Successfully('.$res.')<br>Post ID is '.$postID.'<br>';    }   ////////////  Image Post Attachment Edit  ///////////      $img_attach_content2 = array(                'post_type'  => 'attachment',                   'post_status' => 'inherit',                 'post_title' => $postID,                 'post_name' => $postID,                 'post_parent'  => $postID,                'guid'    => $image_returnInfo['url'],                'post_content'   => '',                'post_mime_type' => 'image/jpg'                 );     $res2 = $client -> query('wp.editPost', 0, $usr, $pwd,      $image_returnInfo['id'], $img_attach_content2);    $postIDimg =  $client->getResponse();        ////////////   POST EDIT  ///////////      $post_content2 = array(                 'post_status' => 'publish', //publish                'wp_post_thumbnail' => $image_returnInfo['id'],                'custom_fields' =>    array( 'key' => '_thumbnail_id', 'value' =>  $image_returnInfo['id'] )             );            $media2= $client->query('wp.editPost',0, $usr, $pwd, $postID, $post_content2);

This is my version, using wp.newPost and wp.editPost, added on WordPress 3.4, that allow the use of custom post types.

require_once("IXR_Library.php.inc");$title = 'My title';$body = 'My body';$category="category1, category2"; // Comma seperated pre existing categories. Ensure that these categories exists in your blog.$keywords="keyword1, keyword2, keyword3";$customfields=array('key'=>'Author-bio', 'value'=>'Autor Bio Here'); // Insert your custom values like this in Key, Value format$title = htmlentities($title,ENT_NOQUOTES,@$encoding);$keywords = htmlentities($keywords,ENT_NOQUOTES,@$encoding);$content = array(    'post_title'=>$title,    'post_content'=>$body,    'post_type'=>'some_custom_post_type',    'post_status' => 'draft', // http://codex.wordpress.org/Post_Status    'mt_allow_comments'=>0, // 1 to allow comments    'mt_allow_pings'=>0, // 1 to allow trackbacks    'mt_keywords'=>$keywords,    'categories'=>array($category),    'custom_fields' => array($customfields));// Create the client object$client = new IXR_Client('http://example.com/xmlrpc.php');$username = "wp_username";$password = "wp_username_password";$params = array(0,$username,$password,$content,true); // Last parameter is 'true' which means post immediately, to save as draft set it as 'false'if (!$client->query('wp.newPost', $params)) {    die('<br/><strong>Something went wrong - '.$client->getErrorCode().' : '.$client->getErrorMessage().'<br >');    }else{       $post_id = $client->getResponse();    echo 'Inserted with id'.$post_id;    $picture = '/full/path/to/pic.jpg';    $content = array(        'name' => basename($picture),        'type' => mime_content_type($picture),        'bits' => new IXR_Base64(file_get_contents($picture)),        true    );    if (!$client->query('metaWeblog.newMediaObject', 1, $username, $password, $content)) {        die('<br/><strong>Something went wrong - newMediaObject'.$client->getErrorCode().' : '.$client->getErrorMessage().'<br >');    }    else    {        $media = $client->getResponse();        $content = array(            'post_status' => 'publish',            'post_thumbnail' => $media['id']        );        if (!$client->query('wp.editPost', 0, $username, $password,  $post_id, $content)) {            die('<br/><strong>Something went wrong editPost - '.$client->getErrorCode().' : '.$client->getErrorMessage().'<br >');        }    }}