WordPress – Custom CSS for all images in website

You can add this code to header.php located in /var/ww/your_wordpress/wp-content/themes/your_theme

img{

-webkit-box-shadow: 13px 7px 28px -1px rgba(0,0,0,0.75);
-moz-box-shadow: 13px 7px 28px -1px rgba(0,0,0,0.75);
box-shadow: 13px 7px 28px -1px rgba(0,0,0,0.75);
}