You can concatenate strings with the result of a php function:
echo '<div id="bg" style="background:url(' . get_stylesheet_directory_uri() . '/assets/img/header_2.jpg);">';
You can concatenate strings with the result of a php function:
echo '<div id="bg" style="background:url(' . get_stylesheet_directory_uri() . '/assets/img/header_2.jpg);">';