Properly adding featured image to theme

The wordpress theme we’re using supported featured images but seemed to use the first image of each post with no prompt to ADD featured image manually. This became a bigger issue when i started using nextgen gallery.

i want to change that, i’ve already added featured image to the posting page. but now i need it to work for the theme.

<div class="container">
    <div class="headergallery">
	<?php $my_query = new WP_Query('category_name=featured&showposts=3'); ?>
	<?php while ($my_query->have_posts()) : $my_query->the_post(); ?>
		<div class="headergalleryimage">
			<?php if (get_post_meta($post->ID, "featuredimage", true) != '') { ?>
			<a href="<?php the_permalink() ?>" title="<?php the_title() ?>">
            <img src="<?php bloginfo('template_url') ?>/timthumb.php?src=<?php echo urlencode(get_post_meta($post->ID, "featuredimage", true)) ?>&w=185&h=185&zc=1" alt="<?php the_title() ?>" />
            </a>
			<?php } elseif (preg_match('/<img([^>]*)>/i', get_the_content(), $match) != FALSE) {
				preg_match('/src=([\'"])([^\'"\1]+)\1/i', $match[1], $imagesrc);
				$url = $imagesrc[2];
			?>
			<a href="<?php the_permalink() ?>" title="<?php the_title() ?>">
            <img src="<?php bloginfo('template_url') ?>/timthumb.php?src=<?php echo urlencode($url); ?>&w=185&h=185&zc=1" alt="<?php the_title() ?>" />
            </a>

That is the section of my header that seems relevant to adding the featured image.

how can i fix this so that it chooses the featured image I’ve set for the post?