I Got the background image working in 3.0.0-0. I will be able to recreate my older themes now !! Attached is a working background image...not real pretty...but can be used as a base...lol.
How To: Add Background Image to TWRP Theme
Open the ui.xml with notepad++ or similar and add the lines arrowed and in brackets in the locations pictured below. The image line can be entered anywhere in the resources area of XML as pictured below
The next part of the XML locates the Image in the theme. Enter the lines in the XML as pictured below.
You must then add the background.png to the "images" folder of the theme you are working on. The Image must match the resolution of the theme xml that you are using. In my case it is 1080x1920.
Rezip you modded theme with 7zip or similar and save it as “your theme name_ui.zip” or similar.
That is it. Install the modded theme as required to be recognized by the TWRP you have install on your device.