I don't think anyone has told you. BUT. the only way is to create your own "cache" but you'd need to create a model with the background images in it.

I won't go into the specifics but you can look here to look for the basics on how to get set up.

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http://www.tophattwaffle.com/configuring-propper-for-steampipe/

then for the picture you will do something similar to this but instead in the mod you just created folder.

http://www.worldofleveldesign.com/categories/wold-members-tutorials/keith_garry/custom-textures-hammer-keithgarry.php

Save your textures in

C:\Program Files (x86)\Steam\steamapps\sourcemods\YOUR MOD\materials\WHAT I WANNA CALL THIS FOLDER

set material type to "monitorscreen" on export to make it glow.

Once you got your model with the images you want created, use the submaterial tool with it the exact way the Caches work. You then have a custom background, but only you will see it unless you choose to share it on Workshop.

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Credit to the respectful creators. NOT BY ME! #READ!

If you can't figure this out, I'm sorry. though THIS is the only way to add backgrounds and custom imagery.

I'm not going to help beyond this. I have a life of my own as well, so please respect that. K thx Bai!