There is no airline in my country that use decoration on its planes, they only use colors to be different than the others.
I remember that in the past, some companies did it, but now, I think that no one decore their plains.
But in Europe, some companies use decoration and all of them have an special thing from the country that they are: flags, for example.
I think that, for airlines, it is preferable to spend money in other things (safety elements, better food, etc.) than in painting... but, as I said, it could be a good idea. It may be a new sport: try to recognise those strange things on the air!