A sweet image of Marcus Aurelius looking badass or contemplative, for black shirts and white.
The shirt is meant for a reminder of stoic philosophy more than an advertisement of it. (I would also consider adding in a short quote, but haven't chosen one yet.)
Attached are images of Marcus Aurelius to get you started. ("Aurelius 8" is artwork I do not own, but I like. Although eyes would be nice! Haha.)