Selectively applying image style to a class
Image styles and HTML classes on images are completely unrelated, so you would have to do some sort of manual hack to achieve this. A very basic effort might be to use hook_preprocess_image_style like so:

function MYTHEME_preprocess_image_style(&amp;$vars) {
  if (array_search('some-class', $vars['attributes']['class']) !== FALSE) {
    $vars['style_name'] = 'some_style';
  }
  elseif ($another_condition) {
    $vars['style_name'] = 'some_other_style';
  }
}