I think that there is actually no way currently, for us to edit the actual images on the car's livery. Every editable image only changes the menus, the only way you could change the sponsors is by changing the livery as a whole (custom liveries). I still think this mod is not worthless and I will at least make the Logos for the menu. So I will keep working on this mod and make custom secondary sponsorships for the menu. But sadly I don't see any way for me to make the logos on the livery work. Maybe if a Modder with more experience and or more info on the matter tries we will come to the conclusion we all hope for.
Until then,
with best regards -"Namydad"