<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:state_checked="true" android:drawable="@drawable/checked"></item>
  <item android:state_checked="false" android:drawable="@drawable/unchecked"></item>
  <item android:drawable="@drawable/unchecked"></item><!-- The default one -->
</selector>

<CheckBox
  android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:button="@drawable/custom_checkbox"
/>