How to change Layout background dynamically in Android?

I have parsed JSON at the time of Splash screen, in which image url is parsed for background image for login screen. Here is sample XMLcode for login screen:

<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
xmlns:tools="http://ift.tt/LrGmb4"
android:id="@+id/loginLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/background_bg"  <!-- I want to change this background dynamically. --> 
android:focusableInTouchMode="true"
android:gravity="center"
tools:context=".activity.LoginActivity" >

<ScrollView
    android:id="@+id/mainScrollView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical" >

 <!-- .... Here is edit text for login inputs and buttuns for singnup and login. -->

    </LinearLayout>
</ScrollView>   
</RelativeLayout>

In above i have placed static image in the background of RelativeLayout but i want to make backgroud as changeble according to image url.

Thanks in advance.