how to make 3D image button transparent and scale the image 40dp by 40dp? android, xml

[this is how my button appears right now:][1]http://ift.tt/1tPPZWv

You can see the button it is a bit different from the background. I want the button to be transparent. I just want the white bluetooth image to show. Here is the code for the button:

 <RelativeLayout 
        android:id="@+id/audio_routing"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:orientation="horizontal">
    <ImageButton 
        android:id="@+id/audio_routing_btn"
        android:contentDescription="@string/audio_routing_desc"
        android:scaleType="fitXY"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:src="@drawable/bluetooth"/>
    </RelativeLayout>

The button is now 40dp by 40dp, but I want the bluetooth image to be 40dp by 40dp. I played around with all the options in:

android:scaleType=""

none of them made the image 40dp by 40dp. So two problems, scale the image on the button to 40dp by 40dp and make the button transparent, (I don't want to see the box)

[this is the bluetooth image that I am using:][1]http://ift.tt/1tPPZWx

I cropped out the white backgroud using gimp.

Thank you