Support lazy load?

I’m afraid it’s not possible. Featured images are inserted using a CSS background property (see the FAQ) and, as far as I know, no lazy load plugin can deal with that.

Is it possible to change the code so that CSS background property is not used?

Yes, it is. Go to includes/nelio-efi-main.php line 59, where the function nelioefi_get_html_thumbnail is defined:

function nelioefi_get_html_thumbnail( $id, $size = false, $attr = array() ) {
  if ( uses_nelioefi( $id ) === false )
    return false;

  $image_url = nelioefi_get_thumbnail_src( $id );
  // ...

  if ( is_feed() ) {
    $style = '';
    if ( isset( $attr['style'] ) )
      $style = 'style="' . $attr['style'] . '" ';
    $html = sprintf( ...
  } else {
    $html = sprintf( ...
  }

  return $html;
}

Change this:

if ( is_feed() ) {

to this:

if ( true ) {

This way, the image will be inserted using the src attribute of an img tag, instead of a CSS background property.

Thanks, worked well. Do you think you will add this as an option for the plugin in the future? This way this sticks when the plugin is updated.

Sure! I can add that as a setting.

Great, thanks!

Another question. By default, your plugin automatically replaces featured images with external image, if there is an external image. Is there a function I can manually call to display external featured image?

What I’m looking to do is add a specific property to the img src tag to stop lazy load on specific images. However, I would need to manually call the external featured image to do that, because I don’t see any other way to selectively disable lazy load.

Our plugin includes a function called: nelioefi_get_thumbnail_src($post_id), which returns the URL of the external featured image used by the post $post_id, or false if no external feat. image is used. I hope this helps!