[Klemm]

Hint: make sure you are labelling the tga file 'background0.tga', not 'background.tga'

That is correct.

The opening splashscreen uses the former, campaigns the latter. You will also need a 'background0.mat' beside the 'background0.tga'. It doesn't matter whether you use 'files>gui' or 'mods>splashscreen>gui'

No, you won't need the 'backround0_ru.mat', since 'background0.tga' is already specified in the standard .mat.
You only need the mat file if you want to give the background .tga another name (anyname.tga).
Then you'll have to change the file name accordingly in the .mat file, but with my method (what i wrote in my last post) you don't even need to extract any files from the sfs archives, only create the correct folder structure in the 'MODS' folder and you're good to go.