The styled image is not displaying
If you go to admin/config/media/file-system what is your public path set as?

If your public path is set as sites/default/files then I think your code is generating the following link:


  http://yourdomain/sites/default/files/sites/all/themes/glossy/logo.png


Try changing your $img_url to the following


  $img_url = drupal_get_path('theme', 'glossy') . '/logo.png';


On another note, is the block rendering at all (just without the image)?  Your implementation of hook_block_view looks incorrect.

I think it should be more like the following:

/**
 * Implementation of hook_block_view().
 */
function glossy_helper_block_view($delta = '') {
  $block = array();

    switch ($delta) {
      case 'glossy_footer_sublogo':
              $img_url = 'public://sites/all/themes/glossy/logo.png';
              $style = 'footer_sublogo';

              $block['subject'] = ''
              $block['content'] = "&lt;img src=" . image_style_url($style, $img_url) . "&gt;",

              );
       break;
    }

  return $block
}


Edit

Change $img_url as I previously mentioned, but rewrite your block content as follows:

$block['content'] = '&lt;img src="' . image_style_url($style, $img_url) . '" /&gt;';


There aren't any quote marks around the image URL that is being returned by the function.