WordPress SimplePress Theme Customization: Adding custom thumbnails to home page slider

I recently worked on a project where I needed the ability to have the navigation thumbnails on the home page slider of the SimplePress WordPress theme to be customizable, not automatically generated off of the featured image as they are by default. Here are the steps I took to accomplish that:

First, install and activate the Multiple Post Thumbnails plugin.

Next, copy and paste this code into your theme’s functions file (be sure to create a child theme of the SimplePress theme before making these customizaations!):

if (class_exists('MultiPostThumbnails')) {
	new MultiPostThumbnails(array(
	'label' => 'Secondary Image',
	'id' => 'secondary-image',
	'post_type' => 'post'
 ) );
 }

Third, open the “includes/featured.php” file of your child theme, and replace this code on lines 109-118:

<?php
	print_thumbnail( array(
	'thumbnail' 	=> $arr[$i]["thumb_small"],
	'use_timthumb' 	=> $arr[$i]["thumbnail"]["use_timthumb"],
	'alttext'		=> $arr[$i]["fulltitle"],
	'width'			=> (int) $width_small,
	'height'		=> (int) $height_small,
	'et_post_id'	=> $arr[$i]['post_id'],
) );
?>

With this:

<?php
if (class_exists('MultiPostThumbnails')) {                              
$t=0;
      while ($t<1) {
          $image_name = 'secondary-image';
          if (MultiPostThumbnails::has_post_thumbnail('post', $image_name)) { 
              $image_id = MultiPostThumbnails::get_post_thumbnail_id( 'post', $image_name, $arr[$i]['post_id'] ); 
          }   
          if ($image_id == "") {
              $image_id = get_post_thumbnail_id( $arr[$i]['post_id'] ); 
          }
              $image_attributes = wp_get_attachment_image_src( $image_id );  ?>
              <img src="<?php echo $image_attributes[0]; ?>" alt="<?php echo $arr[$i]["fulltitle"]; ?>" width="<?php echo $width_small; ?>" height="<?php echo $height_small; ?>">
          <?php
                $t++;
      }                                     
}; 
?>

And there you have it – now you have the ability to choose a secondary featured image for your posts which will be used for the navigation thumbnail in the home page slider when available, otherwise the thumbnail will default to the featured image as usual.