Inokis

This is how I did it in my Inventory Window.

You make a screen item based on a template, example below:
<Screen item = "Window_Texture">
<ScreenID>WindowTexture</ScreenID>
<RelativePosition>true</RelativePosition>
<Location>
<X>163</X>
<Y>44</Y>
</Location>
<Size>
<CX>113</CX>
<CY>192</CY>
</Size>
<Style_VScroll>false</Style_VScroll>
<Style_HScroll>false</Style_HScroll>
<Style_Transparent>false</Style_Transparent>
<TooltipReference></TooltipReference>
<DrawTemplate>WDT_Texture</DrawTemplate>
<Style_Titlebar>false</Style_Titlebar>
<Style_Closebox>false</Style_Closebox>
<Style_Minimizebox>false</Style_Minimizebox>
<Style_Border>false</Style_Border>
<Style_Sizable>false</Style_Sizable>
</Screen>

Then you name the texture file you want to use:
<TextureInfo item="Texture.tga">
<Size>
<CX>256</CX>
<CY>256</CY>
</Size>
</TextureInfo>

Then you need to add Pieces for your screen item:
<Pieces>Window_Texture</Pieces>


Lastly at the very bottom of your document, between </screen> and </xml> copy a WDT. Below is the portion of the WDT you need to change to make this method work:
<WindowDrawTemplate item = "WDT_Texture">
<Background>Texture.tga</Background>