Firstly you need to create a class in css by any name ,not necessary you write everything in body bcs sometime when we use masterpage than we change the content below master page so create a css like:
bg{
background-image:url('../Content/images/utki.jpg');
background-color:#FF6600;
width: 100%;
height: auto;
}
Apply this css any div where u want like
<div class="bg"></div>