nested conditional to check for feature image and custom field?

by

I believe this should do what you want:

<?php
if ( has_post_thumbnail() ) {
    echo '<div id="venue-single-img">';
    the_post_thumbnail();
    if ( get_post_meta($post->ID, 'Venue Image Notation', true) ) {
        echo '<span>' . get_post_meta($post->ID, 'Venue Image Notation', true) . '</span>';
    }
    echo '</div>';
}
?>