Before I started this up, I asked around in Discord what people generally expected quality-wise (Mostly what staff expected, in particular). It was rather obvious that no NSFW or other clearly inappropriate images were allowed, but they further clarified things like no anime, no celebrities, no memes, and so on. Staff also suggested that someone from the community might be able to volunteer to aid with the validating process, but so far no one has come forth on that (and it has honestly not been discussed much). Thusfar, I've been using my own gut to judge whether or not anything has or hasn't been appropriate, and whenever I've had doubts (which so far has only been once), I've double-checked with at least one member of staff to confirm. So Aschent is correct in saying that staff isn't directly involved in the decision making concerning the portraits, but I do ask around whenever I('ll) have doubts and take people's opinions into consideration. I imagine there will be more than a few images that will skirt the quality 'expectations', and will require some discussion.
As for your particular image, Gallows, it was deemed 'too memey', and I agreed, especially since you were ambiguous about even using the image yourself. It seemed to lean too much on the 'one off joke character' side, to me. If you/someone ends up really wanting to use it and writes up a whole bio for it and everything, a longer look will probably be taken. So yes, at the moment I'm basically deciding myself what goes in the pack and what doesn't. Some common sense criteria are in place, but there's definetly still gray areas. That said, one can always discuss in this very thread why they feel a wrong decision has been made. Overall, though, I feel like most people who play on persistent RP worlds like this know what kind of portraits are expected/acceptable.
Hope that clarifies a thing or two about my decision-making process. Once the portrait pack gets to about twenty images, I'll make it public on the Steam Workshop for everyone to use in their single player or wherever and/or do some quality control of their own, or testing (besides my own) to see if their image seems allright in-game and so on.