Hi!
The image showing behind the Nelson text in the screenshot is the game itself, it's to demonstrate what the codex looks like while playing (so it's not an image, but the current scene).
To add individual backgrounds, you'll have to comment out the add "gui/overlay/game_menu.png" in the screen people_nav. Note that if you do this, none of the codex entries (including the welcome page) in the people section will have a background, but assuming you have a custom background for everyone, it's no problem and you can manually add back the generic background too.
You can also skip this part, if you have a slightly transparent game_menu.png (like the default) and you're okay with a slightly dim character image, like in the screenshot.
To add a background to an entry, put add "[filename].png" before the use people_nav line. It's important to add it before, otherwise, the background will show up on top of the menu.
If you want to add back to the generic background under the character art, write add "gui/overlay/game_menu.png" before the line where you add the custom image. Repeat for each entry.
I recommend using images of the same resolution as your game. Makes life easier.
I hope I helped :)