android: how to add an image button?

I am trying to replace my previous ugly text button with an imagebutton. However, after changing the XML file with the following ImageButton code, my application won't even start. Why?

<ImageButton 
android:layout_height="fill_parent"
     android:id="@+id/refresh"
android:src="@drawable/refresh"              
 /> 

From stackoverflow

• It turned out that I forgot the layout_width attribute. It works now.

  <ImageButton 
          android:layout_height="fill_parent"
          android:id="@+id/refresh"
          android:src="@drawable/refresh"     
          android:layout_width="wrap_content"      
      /> 
  

• specify the layout width..

  android:layout_width="wrap_content/*some value ex:50dp*/"