Okay, I have been pondering this (which I didn't fully explain in the first post above) ...bear with me here...I think that there is a time and place for a "smiling Epicurus"...
...but I am now thinking that perhaps that place is in one's imagination. Likely if Epicurus was teaching about the nature of things (atoms and void, etc.) then he had a serious expression. Smiling evolved from primates as a sign of submission, and even now a lecturer or presentor who smiles too much appears less confident.
So I am thinking now that the Epicurus who represents the teachings should be somewhat serious looking.