View difference between Paste ID: 7KMAffdJ and F5EvbeXB

 /**
  * Output the slider.
  *
  * @since 0.1
  */
 function responsive_slider() {
 
 $slide_return = '';
 
 	$slides = new WP_Query( array( 'post_type' => 'slides', 'order' => 'ASC', 'orderby' => 'menu_order', 'posts_per_page' => -1 ) );
 
 	if ( $slides->have_posts() ) :
 
 		$slide_return .= '<div class="responsive-slider flexslider">
 
 			<ul class="slides">';
 
 			while ( $slides->have_posts() ) : $slides->the_post();
 
 				$slide_return .= '<li>
 
 					<div id="slide-' . get_the_ID() . '" class="slide">';
 
 						 global $post;
 
 							 if ( has_post_thumbnail() ) :
 								$slide_return .= '<a href="' . get_post_meta( $post->ID, "_slide_link_url", true ) . '" title="' . the_title_attribute('echo=0') . '" >';
-									$slide_return .= get_the_post_thumbnail( 'slide-thumbnail', array( 'class'	=> 'slide-thumbnail' ) );
+									$slide_return .= get_the_post_thumbnail( $post->ID, 'slide-thumbnail', array( 'class'	=> 'slide-thumbnail' ) );
 								$slide_return .= '</a>';
 							 endif;
 
 						$slide_return .= '<h2 class="slide-title"><a href="' . get_post_meta( $post->ID, "_slide_link_url", true ) . '" title="' . the_title_attribute('echo=0') . '" >' . get_the_title() .'</a></h2>';
 
 					$slide_return .= '</div><!-- #slide-x -->';
 
 				$slide_return .= '</li>';
 
 			 endwhile;
 
 			$slide_return .= '</ul>
 
 		</div><!-- #featured-content -->';
 
 	 endif;
 
 	wp_reset_query();
 
 return $slide_return;
 }