How to leave rounded ImageButton borders?

My XML:

<item>
    <shape>
        <gradient
            android:angle = "270"/>
        <corners
            android:radius = "25dp"/>
        <padding
            android:left="5dp"
            android:top="5dp"
            android:right="5dp"
            android:bottom="5dp" />
    </shape>
</item>

It has the rectangular image. How do I make the image rounded as the border?