Replacing custom field with shortcode

A shortcode like this

[thumbnail height=100 width=100]http://urltoimage.com/image.jpg[/thumbnail]

Would need this type of setup

function content_insert_thumbnail($atts, $content = null){
    global $post;
    extract($atts);
    $thumbnail = get_the_post_thumbnail($post->ID, array($height, $width));
    $content .= $thumbnail; //Places the thumbnail after the content
    return $content; //Never echo a value from a shortcode function.
}
add_shortcode('thumbnail', 'content_insert_thumbnail');

To call this in your theme to insert your thumbnail:

$atts = array(
    'height' => 100,
    'width' => 100
);
content_insert_thumbnail($atts);

This example doesn’t make sense to place in your theme as you can just call the_post_thumbnail() function. However, this example is just to show you how to access the shortcode function (whatever it may be) in your template files.