The cron-job image, as set in viewforum.php and controller/helper.php:
$template->assign_var('RUN_CRON_TASK', '<img src="' . $url . '" width="1" height="1" alt="cron" />');
Renders like this:

<img src="/forum/app.php/cron/cron.task.text_reparser.poll_option" width="1" height="1" alt="cron">

2 issues:

1. CSS `position` is not set, defaulting to `static`, therefore still affects layout (breaks some styles)
2. For some reason, `alt` is set to "cron", when it should be ""; `alt` attribute is supposed to be an accessible alternative representation to the user agent, so should be set to empty string if the element is not supposed to be visible

PR to follow shortly.