Image won't center with android:layout_centerHorizontal - android-layout

Question

my image wont center using the code "android:layout_centerHorizontal" even using gravity wont solve it. Is there any other way or are there wrong inputs in my relativelayout and scroll view.
scale layout bounds
landscape view of with scale layout bounds
screen size 768x1280 nexus api23
I am not having problems with android version 4 maybe this is api related? since my minimum sdk is android version 2?
Here is the code:
<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:fillViewport="true"
android:layout_height="fill_parent">
<RelativeLayout
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="com.capstone.jmilibraryapp.Login">
<EditText
android:layout_width="300dp"
android:layout_height="wrap_content"
android:id="#+id/etUsername"
android:hint="Enter Sr-Code"
android:layout_below="#+id/imageView3"
android:layout_centerHorizontal="true" />
<ImageView
android:layout_centerHorizontal="true"
android:layout_width="500dp"
android:layout_height="200dp"
android:src="#drawable/jmilogo"
android:id="#+id/imageView3"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true" />
<EditText
android:layout_width="300dp"
android:layout_height="wrap_content"
android:id="#+id/etPassword"
android:inputType="textPassword"
android:hint="Password"
android:layout_below="#+id/etUsername"
android:layout_centerHorizontal="true" />
<Button
android:layout_width="300dp"
android:layout_height="wrap_content"
android:id="#+id/bLogin"
android:text="Log In"
android:layout_below="#+id/etPassword"
android:layout_alignLeft="#+id/etPassword"
android:layout_alignStart="#+id/etPassword" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceSmall"
android:text="Log In as Guest"
android:id="#+id/guest"
android:layout_marginTop="20dp"
android:layout_below="#+id/bLogin"
android:layout_centerHorizontal="true" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceSmall"
android:text="Log In as Guest"
android:id="#+id/testinguser"
android:layout_marginTop="20dp"
android:layout_below="#+id/guest"
android:layout_centerHorizontal="true"
/>
</RelativeLayout>
</ScrollView>

Answer 1

Probably your image don´t have the same ratio of your ImageView, you can check it enabling showLayoutBounds in the developer tools in the device. You will see the real size of the imageView.
To solve that you can make you ImageView to be proportionally to your ImageView or ad a scaleType="fitCenter" to your ImageView.

Answer 2

Try below xml this is work with me..
<?xml version="1.0" encoding="utf-8"?>
<ScrollView 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:layout_centerHorizontal="true">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_margin="20dp"
android:orientation="vertical">
<ImageView
android:id="#+id/imageView3"
android:layout_width="wrap_content"
android:layout_height="200dp"
android:layout_gravity="center_horizontal"
android:layout_weight="1"
android:scaleType="fitCenter"
android:src="#drawable/school" />
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginTop="20dp"
android:layout_weight="1">
<EditText
android:id="#+id/etUsername"
android:layout_width="300dp"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:hint="Enter Sr-Code" />
<EditText
android:id="#+id/etPassword"
android:layout_width="300dp"
android:layout_height="wrap_content"
android:layout_below="#+id/etUsername"
android:layout_centerHorizontal="true"
android:hint="Password"
android:inputType="textPassword" />
<Button
android:id="#+id/bLogin"
android:layout_width="300dp"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/etPassword"
android:layout_alignStart="#+id/etPassword"
android:layout_below="#+id/etPassword"
android:text="Log In" />
<TextView
android:id="#+id/guest"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/bLogin"
android:layout_centerHorizontal="true"
android:layout_marginTop="20dp"
android:text="Log In as Guest"
android:textAppearance="?android:attr/textAppearanceSmall"/>
<TextView
android:id="#+id/testinguser"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="#+id/guest"
android:layout_centerHorizontal="true"
android:layout_marginTop="20dp"
android:text="Log In as Guest"
android:textAppearance="?android:attr/textAppearanceSmall"/>
</RelativeLayout>
</LinearLayout>
</ScrollView>