Warning: Cannot modify header information - headers already sent

Question

I've just received this error/warning after trying to upload an image, any ideas what it means and what I can do to fix it?

Warning: Cannot modify header information - headers already sent by (output started at /home/achenn/public_html/wp-content/themes/whskytngfxtrt/functions.php:14) in /home/achenn/public_html/wp-includes/pluggable.php on line 866

Here's my site, live.

functions.php:

<?php

// sidebar functions

if(function_exists('register_sidebar'))
{
    register_sidebar();
}


?>


<?php

// portfolio functions

    add_action('init', 'create_portfolio');
    function create_portfolio() {
        $portfolio_args = array(
            'label' => __('Portfolio'),
            'singular_label' => __('Portfolio'),
            'public' => true,
            'show_ui' => true,
            'capability_type' => 'post',
            'hierarchical' => false,
            'rewrite' => true,
            'supports' => array('title', 'editor', 'thumbnail')
        );
        register_post_type('portfolio',$portfolio_args);
    }
?>

<?php

// custom input- portfolio backend

    add_action("admin_init", "add_portfolio");
    add_action('save_post', 'update_website_url');
    function add_portfolio(){
        add_meta_box("portfolio_details", "Portfolio Options", "portfolio_options", "portfolio", "normal", "low");
    }
    function portfolio_options(){
        global $post;
        $custom = get_post_custom($post->ID);
        $website_url = $custom["website_url"][0];
?>
    <div id="portfolio-options">
        <label>Website URL:</label><input name="website_url" value="<?php echo $website_url; ?>" />     
    </div><!--end portfolio-options-->   
<?php
    }
    function update_website_url(){
        global $post;
        update_post_meta($post->ID, "website_url", $_POST["website_url"]);
    }
?>

<?php 

// detail columns- portfolio backend

add_filter("manage_edit-portfolio_columns", "portfolio_edit_columns");
add_action("manage_posts_custom_column",  "portfolio_columns_display");

function portfolio_edit_columns($portfolio_columns){
    $portfolio_columns = array(
        "cb" => "<input type=\"checkbox\" />",
        "title" => "Project Title",
        "description" => "Description",
    );
    return $portfolio_columns;
}

function portfolio_columns_display($portfolio_columns){
    switch ($portfolio_columns)
    {
        case "description":
            the_excerpt();
            break;              
    }
}
?>

portfolio.php-

<?php
/*
Template Name: Portfolio
*/
?>
<?php get_header(); ?>
    <div id="content">
    <?php 
        $loop = new WP_Query(array('post_type' => 'portfolio', 'posts_per_page' => 10)); 
    ?>
    <?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
    <?php   
        $custom = get_post_custom($post->ID);
        $screenshot_url = $custom["screenshot_url"][0];
        $website_url = $custom["website_url"][0];
    ?>
        <div id="portfolio-item">
        <h1><?php the_title(); ?></h1>
        <a href="<?=$website_url?>"><?php the_post_thumbnail(); ?> </a>
        <?php the_content(); ?>
    </div>
        <?php endwhile; ?>  
        </div><!-- #content -->
<?php get_footer(); ?>