You could either do it this way
<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"
tools:context="com.example.newapp.Firstscreen$PlaceholderFragment" >
<ImageView
android:id="@+id/imageView1"
android:layout_width="match_paren"
android:layout_height="match_paren"
android:scaleType="fitXY"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:src="@drawable/background" />
</RelativeLayout>
OR
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:background="@drawable/background"
tools:context="com.example.newapp.Firstscreen$PlaceholderFragment" >
<!-- CONTENT HERE -->
</RelativeLayout>