evanpyrz on "Modification to Theme to Add Image Link"

Hi there,
I have a general working knowledge of PHP and HTML, but am fairly new to coding for WordPress. Hoping this topic doesn't seem too far-fetched for a forum. What I am trying to do is modify an existing theme I have, to add a URL to a post's image to the HTML rendered. I though I had made the correct modification, however, I am unable to get WordPress to set my $Img variable to the post's data.

This is what I have:

function cro_menuq_func( $atts ) {

    extract( $atts );

    $img = get_the_post_thumbnail( $post_id );
    $op = '<div class="quickiemenu">';

    if (isset($title) && $title != '') {
        $op .= '<a href="'. $img .'"><span class="quickietitle cro_accent">' . stripslashes($title)  . '</span></a>';
    }

    if (isset($desc) && $desc != '') {
       $op .= '<span class="quickiedesc">' . stripslashes($desc)  . '</span>';
    }

    if (isset($price) && $price != '') {
       $op .= '<span class="quickieprice cro_accent">' . stripslashes($price)  . '</span>';
    }

    $op .= '</div>';

    return $op;

}
add_shortcode( 'cro_menuq', 'cro_menuq_func' );

I've contacted the theme's developer as well, but they have yet to get back to me, so I wanted to see if someone on here saw something standing out to them.

I'd be happy to provide reference to this entire file if needed.

Thanks in advance,

Eric