Dear Alain,
In order to keep things simple, could we change the "one different artistic motif by major release" model to "the same artistic motif in all releases" ? With this, it would be easier to propagate artwork improvements to all distributions and the overall CentOS visual identity (as product) would be uniform and coherent.
I personally prefer separate artwork concepts for every major release because these versions should distinguish from each other. Of course we could still change an concept during a major life-cycle if it makes sense (e.g. if in retrospective we realize that a concept is not widely accepted.)
Best Regards Marcus