버튼의 이미지 변경은 drawable에 drawable resource file을 xml 의 형태로 추가하면 됩니다.
예: 아래처럼 하면, 보통상태, 눌렀을 때 등의 상태에서 이미지를 바꿀 수 있습니다.
<?xml version="1.0" encoding="UTF-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_selected="true" >
<bitmap android:gravity="center"
android:src="@drawable/ic_launcher"
android:tileMode="repeat" />
<shape>
<solid
android:color="#008cc6" />
<stroke
android:width="1dp"
android:color="@color/colorPrimaryDark" />
<corners
android:radius="5dp" />
<padding
android:left="0dp" />
</shape>
</item>
<item android:state_enabled="false" >
<bitmap android:gravity="center"
android:src="@drawable/ic_launcher"
android:tileMode="repeat" />
<shape>
<solid
android:color="@color/colorButtonDisabled" />
<stroke
android:width="1dp"
android:color="#19ffffff" />
<corners
android:radius="5dp" />
<padding
android:left="0dp" />
</shape>
</item>
<item>
<bitmap android:gravity="center"
android:src="@drawable/ic_launcher"
android:tileMode="repeat" />
<shape>
<gradient
android:startColor="#00849b"
android:endColor="#00849b"
android:angle="270" />
<stroke
android:width="1dp"
android:color="@color/colorPrimaryDark" />
<corners
android:radius="5dp" />
<padding
android:left="0dp" />
</shape>
</item>
</selector>