Sintaxis de imagen de fondo en CSS si está en la carpeta
/* The first method using an image on your server */
background: url('img/someimage.jpg');
/* The second method using an image hosted on another server */
background: url('http://www.someimage.com/someimage.jpg');
/* A third method that allows the site to determine protocol instead of explicitly defining it */
background: url('//www.someimage.com/someimage.jpg');
Dangerous Dunlin