Managed to find this, should anyone else encounter a similar problem. The page that has this define is /includes/modules/additional_images.php, and the relevant section is:
Code:
// List Box array generation:
$list_box_contents[$row][$col] = array('params' => 'class="additionalImages centeredContent back"' . ' ' . 'style="width:' . $col_width . '%;"',
Just change it to:
Code:
// List Box array generation:
$list_box_contents[$row][$col] = array('params' => 'class="additionalImages centeredContent back"' . ' ' . 'style="height:' . $col_width . '%;"',
and problem solved!