title: Can't add background image
body: <p>I installed Android Studio today.When i tried to add background image to my project it doesnt work.I'm writing this code android:background="@drawable/background".Normally when i wrote android it must show the tips but it doesnt show anything.How can i fix?</p>
  
  <pre><code>    &lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      xmlns:app="http://schemas.android.com/apk/res-auto" android:layout_width="match_parent"
      android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
      android:paddingRight="@dimen/activity_horizontal_margin"
      android:paddingTop="@dimen/activity_vertical_margin"
      android:paddingBottom="@dimen/activity_vertical_margin"
      app:layout_behavior="@string/appbar_scrolling_view_behavior"
      tools:showIn="@layout/activity_main" tools:context=".MainActivity"&gt;
     android:background="@drawable/background"
      &lt;TextView android:text="Hello World!" android:layout_width="wrap_content"
          android:layout_height="wrap_content" /&gt;
  &lt;/RelativeLayout&gt;
  </code></pre>