A picture is worth a thousand words and so a good wordpress theme has to have a lot of images. I usually use the function below which I can call from my theme pages to retrieve an image for an post.

function get_post_image( $iImageNumber = 0, $iSize = "thumbnail", $bPrint = false )
{
	global $post;
	global $wpdb;
	$attachment_id = $wpdb->get_var("SELECT ID FROM $wpdb->posts WHERE post_parent = '$post->ID' AND
          post_status = 'inherit' AND ( post_mime_type = 'image/gif' OR post_mime_type = 'image/jpeg'
          OR post_mime_type = 'image/png' ) AND post_type='attachment' ORDER BY post_date ASC LIMIT 1");

	$pics = wp_get_attachment_url($attachment_id, $size, false);

	// If not image found as an attachment,
	// then look inside the content for embeded images
	if($pics == "")
	{
		$szPostContent = $post->post_content;
		$szSearchPattern = '//';
		preg_match( $szSearchPattern, $szPostContent, $images );
		$pics = $images[1];
	}

	// If still not found then use a default image
	if($pics == "") {
		$loc = get_bloginfo('template_directory');
		$pics = $loc . "/images/post-default-large.jpg";
	}
	if ( $bPrint == true && !empty($pics) ) echo ""; else return $pics;
}