Custom Gamestart Not Loading Assets

[user1679]

I'm trying to add assets such as an image to my custom gamestart but I can't seem to get anything to load.

First, I created the folder structure within my mod's root:

assets\fx\gui\textures\gamestarts

Next, I created a 512x192 image in Photoshop, to match the size of the gamestart thumbnails. I saved this as a DDS file with DTX 1 because I'm not using alpha.
When I set this file as the image property of my gamestart in the XML, it doesn't load but instead shows the default "man in spacesuit". I tried saving it as
PNG and TGA but that also didn't work.

I then tried to package it into a GZ file using 7-zip to match those extracted by the XTools but that also didn't load.

So, I copied one of the existing GZ files, renamed it gamestart_bounty_hunter and set this as the value in my custom gamestarts.xml but it
also does not load. If I change my XML to actually use an existing image for example: image="gamestart_2", it works.

Is it not possible to add custom assets?