There lots of great answers here which actually works. I just what to share the way I do this now with a more modern XML syntax, the constraint view. Trust me constraint view rooks!

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#394BAB"
    tools:context="com.appdomain.app.apppro.ActivityName">

    <ImageView
        android:id="@+id/nice_img"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="0dp"
        android:src="@drawable/green_square"
        android:scaleType="centerCrop"
        android:adjustViewBounds="true"