You should be fine giving barbarians ownership of existing roads / buildings, just follow Atilius's example. Make a back up copy of your export_descr_buildings.txt first, what you are doing is not particularly difficult - it's just good practice to do that anyway.

What they are referring to as 'road images' is just what is displayed in the browser telling you what can be or is already built in a city. If you don't have correct image for that, for barbarians you'll just see something that looks like the roman barracks there instead. The actual road that gets built on the campaign map will work fine.

If you're using BI the barbarian culture already has necessary images for paved_roads (but not highways) in RTW it doesn't, that can be fixed but if you don't have BI you need to unpack the data/packs folders to get access to pictures to copy. Post again if you'd like instructions on doing that (and say which game/s you have).