Steps to reproduce the issue

Activate the module on the frontend.

Expected result

Nice images shown.

Actual result

Frontend Error:

Using $this when not in object context	
/var/www/clients/client5/web20/web/modules/mod_random_image/tmpl/default.php:16

Call stack
#	Function	Location
1	()	JROOT/modules/mod_random_image/tmpl/default.php:16
2	require()	JROOT/modules/mod_random_image/mod_random_image.php:29
3	include()	JROOT/libraries/src/Helper/ModuleHelper.php:201
4	Joomla\CMS\Helper\ModuleHelper::renderModule()	JROOT/libraries/src/Document/Renderer/Html/ModuleRenderer.php:98
5	Joomla\CMS\Document\Renderer\Html\ModuleRenderer->render()	JROOT/libraries/src/Document/Renderer/Html/ModulesRenderer.php:47
6	Joomla\CMS\Document\Renderer\Html\ModulesRenderer->render()	JROOT/libraries/src/Document/HtmlDocument.php:491
7	Joomla\CMS\Document\HtmlDocument->getBuffer()	JROOT/libraries/src/Document/HtmlDocument.php:783
8	Joomla\CMS\Document\HtmlDocument->_renderTemplate()	JROOT/libraries/src/Document/HtmlDocument.php:557
9	Joomla\CMS\Document\HtmlDocument->render()	JROOT/libraries/src/Application/CMSApplication.php:1041
10	Joomla\CMS\Application\CMSApplication->render()	JROOT/libraries/src/Application/SiteApplication.php:780
11	Joomla\CMS\Application\SiteApplication->render()	JROOT/libraries/src/Application/CMSApplication.php:201
12	Joomla\CMS\Application\CMSApplication->execute()	JROOT/index.php:49

System information (as much as possible)

Debian 8, PHP7

Additional comments