I think, if I am at the right place at the document, I have allready done this.
Here is what it says:
<SPLITTER>
<Data DefaultSize="1" />
<Entry Name="HorizontalBackground" Bitmap="Splitter_Background_H.png" Margins="1,1,1,1" DrawMode="REGION_E_HSTRETCH,REGION_E_VSTRETCH" />
<Entry Name="VerticalBackground" Bitmap="Splitter_Background_V.png" Margins="1,1,1,1" DrawMode="REGION_E_HSTRETCH,REGION_E_VSTRETCH" />
<Entry Name="HorizontalHandle" Bitmap="Splitter_Horizontal.png" Margins="1,1,1,1" DrawMode="REGION_E_HTILE,REGION_E_VTILE" />
<Entry Name="VerticalHandle" Bitmap="Splitter_Vertical.png" Margins="1,1,1,1" DrawMode="REGION_E_HTILE,REGION_E_VTILE" />
</SPLITTER>