Can I add my own random splash screens?

[yeikov]

Originally Posted by EJGr.Ost_Caspar

No for splashscreens (sorry, if you don't like them - I know, they are 'special' and doesn't meet everyones taste... but still it was all the same one since PF before), but GUI background was planed to stay user editable. Not sure, why it doesn't work anymore.

It does work, it took me a while to figure out how: we now need to change our custom .mat file name to match this schema:
missions/wide_background_en.mat (for widescreen)
missions/background_en.mat (for 4/3)

en/jp/de... is the gui country you choose in the setup menu, so you can create different .mat files, one for each option. Inside the .mat files you can link to any .tga file you like.