title: flex, change the button image when mouse over/out
body: <p>with following code, i have implemented the image change when mouse hover the button.
  but the image name is hardcoded, which is specified in the style. how can make it parameterized?  i want to reuse this small utility with image name as input parameter.
  all the image used(14 images in my case) will be included in the flex project. 
  i am using flex 3</p>
  
  <p>
  </p>
  
  <pre><code>&lt;mx:Style&gt;  
      .myCustomButton {
          upSkin: Embed(source="jjyxfx.png");
          overSkin:Embed(source="cwgl.png");
         downSkin: Embed(source="cwgl.png");
  
      }
  &lt;/mx:Style&gt;
  
  &lt;mx:Button y="0" width="105" height="107" fillAlphas="[1.0, 1.0, 1.0, 1.0]" x="0" fillColors="[#3AA2D9, #3AA2D9]" styleName="myCustomButton" useHandCursor="true" buttonMode="true"/&gt;
  </code></pre>
  
  <p></p>