Create a new layout in drawable folder and name it custom_radiobutton (You can rename also)
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
<item android:state_checked="false"
android:drawable="@drawable/your_radio_off_image_name" />
<item android:state_checked="true"
android:drawable="@drawable/your_radio_on_image_name" />
</selector>
Use this in your layout activity
<RadioButton
android:id="@+id/radiobutton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:button="@drawable/custom_radiobutton"/>