functions.php

<?php

/**
 * Display Posts default image
 * @see https://displayposts.com/2019/01/03/display-default-image/
 */
function be_dps_default_image( $output, $original_atts, $image, $title, $date, $excerpt, $inner_wrapper, $content, $class, $author, $category_display_text ) {
  if( !empty( $original_atts['image_size'] ) && !empty( $original_atts['default_image'] ) && empty( $image ) ) {
    $default_image_id = intval( $original_atts['default_image'] );
		if ( $original_atts['include_link'] ) {
			$image = '<a class="image" href="' . get_permalink() . '">' . wp_get_attachment_image( $default_image_id, $original_atts['image_size'] ) . '</a> ';
		} else {
			$image = '<span class="image">' . wp_get_attachment_image( $default_image_id, $original_atts['image_size'] ) . '</span> ';
		}
    
    if( !empty( $image ) )
      $output = '<' . $inner_wrapper . ' class="' . implode( ' ', $class ) . '">' . $image . $title . $date . $author . $category_display_text . $excerpt . $content . '</' . $inner_wrapper . '>';
  }
  return $output;
}
add_filter( 'display_posts_shortcode_output', 'be_dps_default_image', 10, 11 );