From Martin's photo we can see that it is a very straight look on the lips of Epicurus. Also he seems to be looking downward -- if you factor in that the statue is placed upon a pedestal then you see that that downward gaze is actually a place of power, as he is looking down at the viewer (the pedestal lifts up the form slightly higher than the viewer). So it seems that this seriousness conveys power.
As for a smile, it can convey comraderie, affection, and enjoyment and as modern people I can see that we do need that conveyed at times, whether in one's imagination or in new artistic works.
And as far as creating more graphics which show Epicurus extolling the teachings, I would appreciate anyone uploading images (both serious or slight smile) which you think would help convey wisdom and the aura of a great teacher. And specifically state those that you think would work in graphics.