Change image style during hook_node_view()
I hate to answer my own question, but this answer pointed me in the right direction. Here's the hook that worked in this case:

/**
 * Implements hook_preprocess_HOOK().
 */
function example_preprocess_image_style(&amp;$variables) {
  if (example_ok()) {
    $variables['style_name'] = 'new_image_style';  
  }
}

function example_ok() {
  global $user;
  // If the logged in user is bob, everything is ok.
  if ($user-&gt;uid &amp;&amp; $user-&gt;name == 'bob') {
    return TRUE;
  }
  return FALSE;
}