Simple Else if Statement

Hi

Thankyou for reading.

I have a small code which displays post thumbnails, I would like to add an else/if statement so if this first method of creating thumbnails fails it create thumbnails using a second method.

Here is what Im using so far to create thumbnails:

<!--Begin post thumbnail-->
<div class="postthumbcon">
<?php
// check if the post has a featured image assigned to it.
if ( has_post_thumbnail() ) {
// get the src of the large size featured image
$src = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large' );
$thumbnailSrc = $src[0];
// output image resized with timthumb
?>
<div class="postthumb">
<a href="<?php the_permalink(); ?>">
<img src="<?php bloginfo('template_directory'); ?>/scripts/timthumb.php?src=<?php echo $thumbnailSrc; ?>&h=125&w=165" alt="">
</a>
</div>
<?php } ?>
</div>
<!--end post thumbnail-->

Here is where I would like to add the Else/if

This next code is the Second method of creating thumbnails:

<?php if( get_post_meta($post->ID, "thumb", true) ): ?>
<div class=”homethumb”>
<img src="<?php bloginfo('template_directory'); ?>/scripts/timthumb.php?src=<?php echo get_post_meta($post->ID, "thumb", $single = true); ?>&h=150&w=150&zc=1" alt="<?php the_title(); ?>" width="150" height="150" />
</div>
<?php else: ?>
<?php endif; ?>

Thankyou again
Kind regards