[david.danaan]

Ok, I figured it out. I found in other topics the explanation for unhooking the function. Here is the code I included in functions.php in my child theme. Note that the thumbnail code is inserted directly before the_content:

// Unhook 
function unhook_thematic_functions() {
// Don't forget the position number if the original function has one

remove_action('attitude_loop_content','attitude_theloop_for_single', 10);

}
add_action('init','unhook_thematic_functions'); // removes the header content by using hook attitude_header

add_action('attitude_loop_content','attitude_child_theloop_for_single', 10);

// Add function to display featured image in single post template

function attitude_child_theloop_for_single() {
	global $post;

	if( have_posts() ) {
		while( have_posts() ) {
			the_post();

			do_action( 'attitude_before_post' );
?>
	<section id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
		<article>

			<?php do_action( 'attitude_before_post_header' ); ?>

			<header class="entry-header">
    			<h1 class="entry-title">
    				<?php the_title(); ?>
    			</h1><!-- .entry-title -->
  			</header>

  			<?php do_action( 'attitude_after_post_header' ); ?>

  			<?php do_action( 'attitude_before_post_content' ); ?>

  			<?php
			if ( has_post_thumbnail() ) {
     			the_post_thumbnail(); } ?>

  			
  			<div class="entry-content clearfix">
    			<?php the_content();
    			if( is_single() ) {
						$tag_list = get_the_tag_list( '', __( ', ', 'attitude' ) );

						if( !empty( $tag_list ) ) {
							?>
							<div class="tags">
								<?php
								_e( 'Tagged on: ', 'attitude' ); echo $tag_list;
								?>
							</div>
							<?php
						}
					}

               wp_link_pages( array( 
						'before'            => '<div style="clear: both;"></div><div class="pagination clearfix">'.__( 'Pages:', 'attitude' ),
						'after'             => '</div>',
						'link_before'       => '<span>',
						'link_after'        => '</span>',
						'pagelink'          => '%',
						'echo'              => 1 
               ) );
               ?>
  			</div>
<?php if(get_the_time( get_option( 'date_format' ) )) { ?>
  			<div class="entry-meta-bar clearfix">	        			
    			<div class="entry-meta">
    				<span class="by-author"><?php _e( 'By', 'attitude' ); ?> <a href="<?php echo get_author_posts_url( get_the_author_meta( 'ID' ) ); ?>"><?php the_author(); ?></a></span> |
    				<span class="date"><a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( get_the_time() ); ?>"><?php the_time( get_option( 'date_format' ) ); ?></a></span> |
    				<?php if( has_category() ) { ?>
             		<span class="category"><?php the_category(', '); ?></span> |
             	<?php } ?> 
    				<?php if ( comments_open() ) { ?>
             		<span class="comments"><?php comments_popup_link( __( 'No Comments', 'attitude' ), __( '1 Comment', 'attitude' ), __( '% Comments', 'attitude' ), '', __( 'Comments Off', 'attitude' ) ); ?></span> |
             	<?php } ?> 		          				
    			</div><!-- .entry-meta -->
    		</div>

  			<?php 

  			do_action( 'attitude_after_post_content' );
			 }
  			do_action( 'attitude_before_comments_template' ); 

         comments_template(); 

         do_action ( 'attitude_after_comments_template' );

         ?>

		</article>
	</section>
<?php
			do_action( 'attitude_after_post' );

		}
	}
	else {
		?>
		<h1 class="entry-title"><?php _e( 'No Posts Found.', 'attitude' ); ?></h1>
      <?php
   }
}