<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/checkbox_selected" android:state_checked="true" />  
    <item android:drawable="@drawable/checkbox_unselected" android:state_checked="false" />  

    <item android:drawable="@drawable/checkbox_unselected" />
</selector>