If you're going the XML route, you can add the image file tag like this:
Code:
<Texture name="MyAddonTexture" textureFile="MyAddonName/media/myTexture.dds">
Along with the myriad of other tags you can find here:
http://wiki.esoui.com/UI_XML
I know it works with way with texture defs as I do it myself, might work as well for "Backdrop" types, etc.