<?php 
// Render a specific extrafield
if( isset( $this->item->extraFields->EXTRAFIELDALIASHERE->value ) && ( $this->item->extraFields->EXTRAFIELDALIASHERE->value !== '') ) 
{ ?>

	<div id="ID_WHICH_EF_IS_RENDERED" data-bgimage="<?php echo $this->item->extraFields->EXTRAFIELDALIASHERE->value; ?>"></div>
	
<?php } ?>

<script>
(function($)){
	$(document).ready(function(){

		if( $('#ID_WHICH_EF_IS_RENDERED').length ) {

			// add the data-image="EXTRAFIELD_VALUE" on the template.
			var bgImage = $('#ID_WHICH_EF_IS_RENDERED').data('bgimage');

			// Set the desired element's css style
			$('ELEMENT_YOU_NEED_TO_SET_BG').css('background-image', bgImage);

		}
	});
})(jQuery);
</script>