Solution:
For this you can use a PHP function get_stylesheet_directory_uri() to help you set a relative path.
get_stylesheet_directory_uri(): Retrieves stylesheet directory URI for the current theme/child theme. Checks for SSL.
Put this PHP inside your HTML:
<?php echo get_stylesheet_directory_uri(); ?>
it will echo out your current stylesheet directory.
After that compare the image path and stylesheet path and do something like this:
<img src="<?php echo get_stylesheet_directory_uri(); ?>/images/aternus.png" alt="" width="" height="" />
or, after finding out the sylesheet path you can write a relative path for the image in CSS like this:
background-image: url('./../uploads/2017/12/image.png');
// explanation: /. means current folder, ../ means one folder back (up)