sauramon It's declared in index.php file of template:
/templates/samba_spa/index.php
.son_image_left {background:transparent url(<?php if(strrpos($s5_custom23back,"/") <= 0) {echo $LiveSiteUrl; echo "images/";} echo $s5_custom23back; ?>) no-repeat;}
.son_image_right {background:transparent url(<?php if(strrpos($s5_custom45back,"/") <= 0) {echo $LiveSiteUrl; echo "images/";} echo $s5_custom45back; ?>) no-repeat right;}
Which loads value in Theme settings in backend:
