Set Background Image on a ContentPlaceHolder !!

Question

Hi, 


   I have a child Login.aspx with Master page "Site.Master". I want that every time, user comes on Login page, then different images should be shown. 
In my Login.aspx :
<%@ MasterType VirtualPath="~/Site.Master" %>


<%@ Register Src="~/Account/OpenAuthProviders.ascx" TagPrefix="uc" TagName="OpenAuthProviders" %>


<asp:Content runat="server" ID="BodyContent" ContentPlaceHolderID="MainContent">



In Code-Behind :
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                GetAllBackgroundImages();
            }
            if (bgfiles != null)
            {
                Random r = new Random();
                int index = r.Next(bgfiles.Length);
                string imgFile = bgfiles[index];
                this.Master.ContentBackground = imgFile;
            }

In site.Master :
        <div id="masterContent" class="container body-content" runat="server">
            <asp:ContentPlaceHolder ID="MainContent" runat="server">
            </asp:ContentPlaceHolder>
            <hr />



site.Master code Behind :
        public string ContentBackground
        {
            get
            {
                return "";
            }
            set
            {
                masterContent.Style.Add("background-image", "url('" + value + "')");
            }
        }

With the above code, the images are collected in GetAllBackgroundImages();, a image is also selected and this.Master.ContentBackground = imgFile; is also called, but their is no image seen on the Login page. 

Any idea where am I going wrong ?? Any help is highly appreciated.

Thanks