Enviar publicación y cargar imagen desde el front-end

Si está hablando de la respuesta, publiqué aqui es simplemente cargar el archivo en un iframe para lograr "Ajax like".

Ahora, si ya tiene un formulario que maneja el envío de la publicación, simplemente puede agregar la entrada del campo del archivo de carga en algún lugar de su formulario:

<form ...
...
<input type="file" name="thumbnail" id="thumbnail">
...
...
</form>

asegúrese de que su formulario tenga el atributo enctype="multipart/form-data" .

luego en la secuencia de comandos de procesamiento de formularios después de crear la publicación (suponiendo que está utilizando wp_insert_post(); ) mantener la ID de la publicación en una nueva var:

$new_post = wp_insert_post($post_array);

y después de eso agregue:

            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);
            }

y tu imagen se cargará y se guardará como miniatura de publicación.

Marca HTML:

 <p>
   <label for="custom-upload">Upload New Image:</label>
   <input type="file" tabindex="3" name="custom-upload" id="custom-upload" />
 </p>
 <?php
  /*Retrieving the image*/
  $attachment = get_post_meta($postid, 'custom_image');

  if($attachment[0]!='')
  {
   echo wp_get_attachment_link($attachment[0], 'thumbnail', false, false);
  }

 ?>

Cargando la imagen:

<?php
global $post; /*Global post object*/
$post_id = $post->ID; /*Geting current post id*/
$upload = $_FILES['upload']; /*Receive the uploaded image from form*/
add_custom_image($post_id, $upload); /*Call image uploader function*/

function add_custom_image($post_id, $upload)
{
 $uploads = wp_upload_dir(); /*Get path of upload dir of wordpress*/

 if (is_writable($uploads['path']))  /*Check if upload dir is writable*/
 {
  if ((!empty($upload['tmp_name'])))  /*Check if uploaded image is not empty*/
  {
   if ($upload['tmp_name'])   /*Check if image has been uploaded in temp directory*/
   {
    $file=handle_image_upload($upload); /*Call our custom function to ACTUALLY upload the image*/

    $attachment = array  /*Create attachment for our post*/
    (
      'post_mime_type' => $file['type'],  /*Type of attachment*/
      'post_parent' => $post_id,  /*Post id*/
    );

    $aid = wp_insert_attachment($attachment, $file['file'], $post_id);  /*Insert post attachment and return the attachment id*/
    $a = wp_generate_attachment_metadata($aid, $file['file'] );  /*Generate metadata for new attacment*/
    $prev_img = get_post_meta($post_id, 'custom_image');  /*Get previously uploaded image*/
    if(is_array($prev_img))
    {
     if($prev_img[0] != '')  /*If image exists*/
     {
      wp_delete_attachment($prev_img[0]);  /*Delete previous image*/
     }
    }
    update_post_meta($post_id, 'custom_image', $aid);  /*Save the attachment id in meta data*/

    if ( !is_wp_error($aid) ) 
    {
     wp_update_attachment_metadata($aid, wp_generate_attachment_metadata($aid, $file['file'] ) );  /*If there is no error, update the metadata of the newly uploaded image*/
    }
   }
  }
  else
  {
   echo 'Please upload the image.';
  }
 }
}

function handle_image_upload($upload)
{
 global $post;

        if (file_is_displayable_image( $upload['tmp_name'] )) /*Check if image*/
        {
            /*handle the uploaded file*/
            $overrides = array('test_form' => false);
            $file=wp_handle_upload($upload, $overrides);
        }
 return $file;
}
?>