I have a custom field which saves the URL of an image that is uploaded. This is achieved by obtaining the medium size like this:

$image = get_field('fl_image');
$my_thumb = $image['sizes']['medium'];

The output of the custom field is simply the URL of the medium size picture. e.g.


How can I set this as a featured image when a post is saved/updated and the featured image is empty?

  • Featured images are stored by saving a post meta field with key _thumbnail_id and the value is an attachment ID, so you would need to save your image properly as an attachment for things to work out. There's an easy way out, possibly, if you control your theme and can edit the code. You could check for a featured image and if there isn't one, display your image instead. If that's an acceptable alternative, let me know and I'll knock up some code. Oct 14, 2016 at 12:29
  • I just re-read your question. If your field saves the URL of an image as you state then you wouldn't have any image sizes set like that. Can you expand your question to explain exactly how you have your field set up? Oct 14, 2016 at 12:31
  • This is an ACF image field and I have the option to upload an image and save the output of this field as a URL of the image. At the moment I upload the image, the medium size is automatically created in my uploads folder but not attached to the post, as you are stating.
    – JoaMika
    Oct 14, 2016 at 12:45
  • Being attached to the post and being an attachment are different things. What do you get if you add the line print_r($image) to your code? Oct 14, 2016 at 12:47
  • sorry for being slow.. how can I run print_r($image) and see the output?
    – JoaMika
    Oct 14, 2016 at 12:54

1 Answer 1


Hook 'acf/save_post' after ACF saves the $_POST['acf'] data. Then check to see if the post has_post_thumbnail and set_post_thumbnail if it doesn't.

function save_acf_image_to_post_thumbnail( $post_id ) {

    $image = get_field( 'fl_image' );

    if ( ! empty ( $image ) ) {

        if ( ! has_post_thumbnail( $post_id ) ) {

            $image_id = isset( $image[ 'id' ] ) ? $image[ 'id' ] : ( isset( $image[ 'ID' ] ) ? $image[ 'ID' ] : '' );

            if ( ! empty ( $image_id ) ) {

                set_post_thumbnail( $post_id, $image_id );

add_action( 'acf/save_post', 'save_acf_image_to_post_thumbnail', 20 );

This assumes the Return Value is an Image Object.

It's also possible to trick has_post_thumbnail and post_thumbnail_html to render your custom field Image URL when the metadata is missing.

Spoof has_post_thumbnail:

function filter_post_metadata( $value, $object_id, $meta_key, $single ) {

    if ( $meta_key === '_thumbnail_id' && ! $value ) {

        return empty ( get_field( 'fl_image' ) ) ? $value : - 1;

    return $value;

add_filter( 'get_post_metadata', 'filter_post_metadata', 20, 4 );

Doctor post_thumbnail_html:

function filter_post_thumbnail_html( $html, $post_ID, $post_thumbnail_id, $size, $attr ) {

    $image = get_field( 'fl_image' );

    if ( ! empty( $image ) && (empty( $post_thumbnail_id ) || $post_thumbnail_id === -1) ) :

        $html = sprintf( '<img width="%s" height="auto" src="%s" class="attachment-post-thumbnail size-post-thumbnail wp-post-image" >', "100%", esc_url( $image)  );


    return $html;

add_filter( 'post_thumbnail_html', 'filter_post_thumbnail_html', 20, 5 );

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