I have performed an upgrade to the latest Snow Leopard and now to Mountain Lion (10.8.1). Post upgrade my desktop background isn't no longer changing. The background will not change when I try and select a different background via system preferences. Even if I reset it to the default background the same behavior persists. My system has exhibited this problem for almost 7 months now, I have this ugly plain blue background I cannot look at any more. Can anyone offer a solution that might resolve this?