Using theme_preprocess_node i'm trying to add a class to all images on the page
From what I can tell, hook_preprocess_node() won't let you add both classes and title attributes to images from image fields. Try using hook_preprocess_field() as in the example below:

function myTheme_preprocess_field(&amp;$variables) {
  if (($variables['element']['#field_name'] == 'field_about_image') &amp;&amp; ($variables['element']['#bundle'] == 'basic_page_with_pic')) {
    $variables['items'][0]['#item']['title'] = t('This is the title text that will display when hovering.');
    $variables['items'][0]['#item']['attributes']['class'][] = 'class1';
    $variables['items'][0]['#item']['attributes']['class'][] = 'class2';
  }
}


Be sure to clear your cache.