Table sort indicators are now rendered with CSS via twig

template_preprocess_tablesort_indicator() was removed and its logic was moved to tablesort-indicator.html.twig.

Drupal 7

Copy the theme function theme_tablesort_indicator() from includes/theme.inc to your custom theme's settings.php file and provide a path to your theme's folder.

// MY_THEME/settings.php
function MY_THEME_tablesort_indicator($variables) {
  // Get the path to my theme.
  $path = drupal_get_path('theme', 'MY_THEME');
  if ($variables['style'] == "asc") {
    return theme('image', array('path' => $path . '/icons/arrow-asc.png', 'width' => 13, 'height' => 13, 'alt' => t('sort ascending'), 'title' => t('sort ascending')));
  }
  else {
    return theme('image', array('path' => $path . '/icons/arrow-desc.png', 'width' => 13, 'height' => 13, 'alt' => t('sort descending'), 'title' => t('sort descending')));
  }
}

Drupal 8

Override the following lines to your theme's CSS file and add your new images if need be(could be now done without images)

.tablesort--asc {
  background-image: url(../icons/twistie-down.svg);
}
.tablesort--desc {
  background-image: url(../icons/twistie-up.svg);
}

Also:
SVG Icons have now replaced the existing PNG images used for up and down arrows in the sort order UIs. This helps immensely when viewing on high-resolution displays and mobile devices, since SVGs are resolution-independent.