Need help with if or elseif-statement

Hey all,

I am currently editing a theme (Volt) that uses the TimThumb script. I am trying to make the script 1) display an attached thumbnail if there is one 2) if not, display a thumbnail specific to that category, and 3) if no image is found for the category, display a default image.

Here is part of the current script:

if ( has_post_thumbnail()) {
$img_src = wp_get_attachment_image_src( get_post_thumbnail_id($GLOBALS['post']->ID), '');
$thumbnail = $img_src[0];
}
else { $thumbnail = '';
$default_thumb = $bloginfo.'/images/post_thumb.jpg';
$thumbnail = ( $thumbnail == '' ) ? $default_thumb : $thumbnail;
}

As you can see, 1) and 3) are taken care of in the default script. My issue is that I am a beginner in php and don’t know how/where to put the 2) statement.

Please help!