Here is source code of the Program to change the background of your activity.The Android Java project is successfully compiled and run on Windows system using Eclipse. The program output is also shown below.

Both the codes Main Activity as well as its corresponding layout code Xml are given below.


package com.example.hello_android_sanfoundry;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

}

This is the Xml code.
In order to set image as the background of your activity place the image in the /res/drawable-hdpi and click on Project and then select clean to clean the current project this will load the image in your current working project , in this Application i have set the background using background.png.
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".MainActivity"
android:background="@drawable/background"
>
<TextView
android:id="@+id/textView2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/textView1"
android:layout_alignParentTop="true"
android:layout_marginTop="18dp"
android:text="@string/hello_sanfoundry_android" />

<TextView
android:id="@+id/textView1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentTop="true"
android:layout_marginTop="53dp"
android:text="This is a Large Text View"
android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/textView1"
android:layout_below="@+id/textView1"
android:layout_marginTop="16dp"
android:text="This is Medium Text View"
android:textAppearance="?android:attr/textAppearanceMedium" />

<TextView
android:id="@+id/textView4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/textView3"
android:layout_below="@+id/textView3"
android:layout_marginTop="14dp"
android:text="Small Text"
android:textAppearance="?android:attr/textAppearanceSmall" />

<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/textView4"
android:layout_below="@+id/textView4"
android:layout_marginTop="16dp"
android:text="Button" />

<ToggleButton
android:id="@+id/toggleButton1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/button2"
android:layout_alignBottom="@+id/button2"
android:layout_alignParentRight="true"
android:text="ToggleButton" />

<Button
android:id="@+id/button2"
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignTop="@+id/button1"
android:layout_centerHorizontal="true"
android:text="Small Button" />

<CheckBox
android:id="@+id/checkBox1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignRight="@+id/toggleButton1"
android:layout_below="@+id/toggleButton1"
android:layout_marginTop="93dp"
android:text="CheckBox" />

<CheckedTextView
android:id="@+id/checkedTextView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBaseline="@+id/checkBox1"
android:layout_alignBottom="@+id/checkBox1"
android:layout_alignLeft="@+id/button1"
android:text="CheckedTextView" />

<RadioButton
android:id="@+id/radioButton1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/checkedTextView1"
android:layout_below="@+id/checkBox1"
android:layout_marginTop="34dp"
android:text="RadioButton" />

<RadioButton
android:id="@+id/radioButton2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="@+id/radioButton1"
android:layout_below="@+id/radioButton1"
android:text="RadioButton" />

</RelativeLayout>