I am pretty sure ZombiesRus term "Map Back" is just what I described above. Finding the brush links in the entity and using Find in Files and locate the brush and then the texture sheet they're on.
Ships entities are layed out differently but all the link are basically the same, just in different spots.
picture "CAPITALSHIPPICTURE_PHASEBATTLESHIP" <--big one bottom middle
Lines 93, 94, 95
Different ships will have them on different lines but just search the sheet for
mainViewIcon, picture, hudIcon, smallHudIcon and infoCardIcon
Take a look at Window\LoadScreen.brushes
Notice at the top it has the line
This means only the picture used is loaded into the ram so you can actually use lots of full screen wallpaper images and create a new brush or change an existing brush to point at a new image and place the image in the textures folder.
If you do use full screen loading images you will need to learn how to start editing the .window files which are not that complicated once you start experimenting.