Recently i was keeping Image as background in master page of one of my persnal project . Firsly i was implementing the direct approach to do this task. But in master page, directly we cant keep the image as background image.
Here is some trick to do this task
Steps 1: Create the master page as per as your requirement
Step2 : In div keep the img background syntax like this
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Site1.master.cs" Inherits="Imgae_BackGround_MasterPage.Site1" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
<body>
<form id="form1" runat="server">
<div style="background-image: url('<%= imgPath %>') " >
<br />
<br />
<b style="font-style: italic; color: #fff"> This is the Header menu part of master page</b>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server"></asp:ContentPlaceHolder>
<b style="font-style: italic; color: #fff"> This is the Footer menu part of master page</b>
<br />
<br />
</div></form>
</body>
</html>
Step3: Write the code in code behind of master page like this
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;namespace Imgae_BackGround_MasterPage
{
public partial class Site1 : System.Web.UI.MasterPage
{
public string imgPath = System.Web.VirtualPathUtility.ToAbsolute("~/Img/Hydrangeas.jpg");protected void Page_Load(object sender, EventArgs e)
{}
}
}
Step4: Now create the content page and compile it.
I hope it will help to some one
Chandradev's Blog 
good dude
Thanks.