You'll need GIMP (and the dds plugin),, Photoshop.or any editor that is able to handle the dds format. The four white pixels are to help identify the centre. You want to fit your logo within the black area. Underscore's post covers the basic premise of how it needs to be done but if you have no knowledge of how to mod ETS2 in the first place it might not be clear so, I've made a simple package with a basic texture to help.
It is but explaining it from step one of unpacking game files through to step X of adding the mod you created to the game would require quite a wall of text and a lot of the information exists on the internet already.
