progressbar 图片上下显示

<SeekBar
   android:visibility="invisible"
  android:id="@+id/skbProgress"
     android:layout_width="fill_parent"
         android:layout_height="21dip"
  android:layout_marginTop="120dip"
  android:layout_marginRight="30dip"
  android:layout_marginLeft="30dip"
  android:thumb="@drawable/slide"
    android:progressDrawable="@drawable/seekbar_img"
   style="?android:attr/progressBarStyleHorizontal"     
       android:progress="0"
    android:maxHeight="9dip"
    android:minHeight="9dip" 
  
     android:layout_gravity="center_vertical"
/>    

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
<item android:id="@+android:id/background"  android:drawable="@drawable/pdbg"   />
<item android:id="@+android:id/progress" android:drawable="@drawable/pdbg2"/>
</layer-list>