The picture is more typical for valve seals issue. This is the first idea that comes. Rings issue may be too, but less probable. As for external causes, I would not exclude a gasoline quality: bad composition or, may be, even dilution with a gasoil.
Irrespective of the cause, I would try slightly hotter plugs as a temporary solution. Besides, in case of high deposits it's preferable to use plugs with one-ground electrode.