[android] how to change background image of button when clicked/focused?

Its very easy to implement . For that you need to create a one xml file(selector file) and put it in drawable folder in res. After that set xml file in button's background in your layout file.

button_background_selector.xml

<?xml version="1.0" encoding="UTF-8"?>
<selector
  xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_focused="true" android:state_pressed="false" android:drawable="@drawable/your_hover_image" />
    <item android:state_focused="true" android:state_pressed="true" android:drawable="@drawable/your_hover_image" />
    <item android:state_focused="false" android:state_pressed="true" android:drawable="@drawable/your_hover_image"/>
    <item android:drawable="@drawable/your_simple_image" />
</selector>

Now set the above file in button's background.

<Button
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content"
    android:textColor="@color/grey_text"
    android:background="@drawable/button_background_selector"/>