fall back images for feature post

hi im using the following to show the first image in a post if no feature image is set

<?php if (  (function_exists('has_post_thumbnail')) && (has_post_thumbnail())  ) {
  echo get_the_post_thumbnail($post->ID);
} else {
   echo main_image();
} ?>

which calls

//function to call first uploaded image in functions file
function main_image() {
$files = get_children('post_parent='.get_the_ID().'&post_type=attachment
&post_mime_type=image&order=desc');
  if($files) :
    $keys = array_reverse(array_keys($files));
    $j=0;
    $num = $keys[$j];
    $image=wp_get_attachment_image($num, 'large', true);
    $imagepieces = explode('"', $image);
    $imagepath = $imagepieces[1];
    $main=wp_get_attachment_url($num);
		$template=get_template_directory();
		$the_title=get_the_title();
    print "<img src='$main' alt='$the_title' class='frame' />";
  endif;

id like to take this a bit further and

1. if the first image is displayed have it match the Feature image thumbnails size, other wise its too big.

2. Add a further condition that display a default image if none of the others exist, something like

<?php if (  (function_exists('has_post_thumbnail')) && (has_post_thumbnail())  ) {
  echo get_the_post_thumbnail($post->ID);
} else {
   echo main_image();
} 

another  else if main image doesnt exist then show

<img src="<?php bloginfo('template_directory'); ?>/images/default.jpg" alt="<?php the_title(); ?>" />

?>

but im stumped now!!

ayonne got any ideas, many thanks