Realistically, this may be related to our upgrade, but when we recently upgraded from 1.x to 2.0.17 the *new* GIF indicating new posts in the forum was showing a broken image. A quick look at the URL spotted it: .../Themes/NightBreeze20/images//new.gif
Backtracking through the theme, the broken variable was @settings['lang_images_url'], set in load.php at line 1779,
$settings['lang_images_url'] = $settings['images_url'] . '/' . (!empty($txt['image_lang']) ? $txt['image_lang'] : $user_info['language']);
I did a global search of the 2.0.17 installation files and didn't find 'image_lang' set anywhere, so the quick fix was to add
$txt['image_lang'] = 'english';
at line 14 of /Themes/default/languages/index.english.php
The core & default themes should have the same broken *new* GIF on a fresh 2.0.17 install, as they have the same chunks of code as the theme we're using.