How to change image style using hook_views_pre_render()
I gave up on hook_views_pre_render and wound up meeting my requirements with template_preprocess_image_formatter as follows:

function &lt;TEMPLATE_NAME&gt;_preprocess_image_formatter(&amp;$variables) {
  $styles = array('thumbnail', 'medium', 'large');
  if ($variables['image']['#style_name'] === 'medium') {
    $variables['image']['#style_name'] = $styles[array_rand($styles)];
  }
}