hook_form_FORM_ID_alter in theme file to toggle field visibility
After a lot of banging on this, I found the right combo.  We don't have a custom admin theme, so I ended up moving this code in to a custom module.

The important part I was missing was [value] in ':input[name="field_news_topstory[value]"]' => ['checked' => TRUE]  

use Drupal\Core\Form\FormStateInterface;

function mymodule_form_node_news_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
    $form['field_news_top_story_image']['#states'] = [
       'visible' =&gt; [
         ':input[name="field_news_topstory[value]"]' =&gt; ['checked' =&gt; TRUE]
       ]
   ];
}