Display Gallery Images from Custom Post Type

You can alter the foreach loop as follows,

 foreach( $gallery['ids'] as $attachment_id )
 {
       $image_attributes = wp_get_attachment_image_src( $attachment_id );
       if( $image_attributes ) {

           $src = isset($image_attributes[0])? $image_attributes[0] : '';

            if(!empty($src)) {
                ?>

             <a href="https://wordpress.stackexchange.com/questions/118897/<?php echo $src; ?>" rel="prettyPhoto[pp_gal]"><img src="https://wordpress.stackexchange.com/questions/118897/<?php echo $src; ?>"  alt="Gallery image"  /></a>

            <?php
            }
       }
  }

You can specify required dimension, as second parameter for , ‘wp_get_attachment_image_src()’ function.