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

use this code create xml file in drawable folder name:button

<?xml version="1.0" encoding="utf-8"?>
  <selector xmlns:android="http://schemas.android.com/apk/res/android">
  <item 
     android:state_pressed="true" 
     android:drawable="@drawable/buutton_pressed" />
  <item 
     android:drawable="@drawable/button_image" />
</selector>

and in button xml file

 android:background="@drawable/button"

you can implement in a xml file for this as follows:

<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_focused="true" android:drawable="@drawable/your_imagename_while_focused"/>
<item android:state_pressed="true" android:drawable="@drawable/your_imagename_while_pressed" />
<item android:drawable="@drawable/image_name_while_notpressed" />  //means normal
</selector>

now save this xml file in drawable folder and name it suppos abc.xml and set it as follows

 Button tiny = (Button)findViewById(R.id.tiny);
 tiny.setBackgroundResource(R.drawable.abc);

Hope it will help you. :)

Sorry this is wrong.

For changing background color/image based on the particular event(focus, press, normal), you need to define a button selector file and implement it as background for button.

For example: button_selector.xml (define this file inside the drawable folder)

<?xml version="1.0" encoding="utf-8"?>
 <selector xmlns:android="http://schemas.android.com/apk/res/android">
     <item android:state_pressed="true"
           android:color="#000000" /> <!-- pressed -->
     <item android:state_focused="true"
           android:color="#000000" /> <!-- focused -->
     <item android:color="#FFFFFF" /> <!-- default -->
 </selector>

    <!-- IF you want image instead of color then write 
android:drawable="@drawable/your_image" inside the <item> tag -->

And apply it as:

 <Button
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:drawable="@drawable/button_selector.xml" />

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"/>