As you can't use android:text I recommend you to use a normal button and use one of the compound drawables. For instance:
<Button
android:id="@+id/buttonok"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:drawableLeft="@drawable/buttonok"
android:text="OK"/>
You can put the drawable wherever you want by using: drawableTop, drawableBottom, drawableLeft or drawableRight.
UPDATE
For a button this too works pretty fine. Putting android:background is fine!
<Button
android:id="@+id/fragment_left_menu_login"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/button_bg"
android:text="@string/login_string" />
I just had this issue and is working perfectly.