how to add background image to activity?

using theme or ImageView ?

use the android:background attribute in your xml. Easiest way if you want to apply it to a whole activity is to put it in the root of your layout. So if you have a RelativeLayout as the start of your xml, put it in here:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/rootRL"
    android:orientation="vertical" 
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" 
    android:background="@drawable/background">
</RelativeLayout>

You can set the "background image" to an activity by setting android:background xml attributes as followings:

(Here, for example, Take a LinearLayout for an activity and setting a background image for the layout(i.e. indirectly to an activity))

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:id="@+id/LinearLayout01" 
			  android:layout_width="fill_parent" 
			  android:layout_height="fill_parent" 
			  xmlns:android="http://schemas.android.com/apk/res/android"
			  android:background="@drawable/icon">
 </LinearLayout>

Place the Image in the folder drawable. drawable folder is in res. drawable have 5 variants drawable-hdpi drawable-ldpi drawable-mdpi drawable-xhdpi drawable-xxhdpi

We can easily place the background image in PercentFrameLayout using the ImageView. We have to set the scaleType attribute value="fitXY" and in the foreground we can also display other view's like textview or button.

 <android.support.percent.PercentFrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        >
        <ImageView
            android:src="@drawable/logo"
            android:id="@+id/im1"
            android:scaleType="fitXY"
            android:layout_height="match_parent"
            android:layout_width="match_parent"/>
<EditText android:layout_gravity="center_horizontal"
        android:hint="Enter Username"
        android:id="@+id/et1"
        android:layout_height="wrap_content"
        app:layout_widthPercent="50%"
        app:layout_marginTopPercent="30%"
        />
<Button
    android:layout_gravity="center_horizontal"
    android:text="Login"
    android:id="@+id/b1"
    android:layout_height="wrap_content"
    app:layout_widthPercent="50%"
    app:layout_marginTopPercent="40%"/>
</android.support.percent.PercentFrameLayout>

Nowadays we have to use match_parent :

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    android:orientation="vertical" 
    android:background="@drawable/background">
</RelativeLayout>

ez way : copy your picture in this location: C:\Users\Your user name\AndroidStudioProjects\yourProjectName\app\src\main\res\drawable

and write this code in your xml file:

 <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@drawable/background">

</RelativeLayout>

in line 7 write your file name instand of background

and Tamam...

and dont forget to clean your project after writing these lines you`ll a get an error in your xml file until you´ve cleaned your project in eclipse: Project->Clean...