I don't know if there is any Spanish airline that has decorative planes, but I know one air company from Ireland which has a harp on its airplanes: Ryanair.
This harp symbolise Ireland because it's a musical instrument to play celtic music.
I like it very much because this symbol is on the bottel of the most famous iris bier: Ginness.
Last year, I have visited the Ginness Museum in Dublin. The building of this Museum seems a big bier glass and on the top of it there's a bar with a fantastic views of the city.
I have enjoyed it very much!