Get the current theme directory, good for use in wordpress templates.

<?php echo get_template_directory_uri(); ?>

Usage Example

To link to an image located in the theme directory

<img src='<?php echo get_template_directory_uri(); ?>/img/image.jpg'>

background image css inline

.ebook-slide {
	background:url("<?php echo get_template_directory_uri(); ?>/img/image.jpg") center no-repeat;
	background-size:cover;
	height:600px;	
}