@tuatarara maybe it's an idea to use a hashtag for it? Something like #CritiqueWelcome or perhaps more specifically #ArtCritiqueWelcome?

Could also create a Guppe group, but I fear that having to rely on another instance/service for it could be another barrier to enter, as well as another potential point of failure.

I wouldn't call myself a professional, beginning nor otherwise, nor do I have formal training, but I wouldn't mind some constructive critique on some of my work. Getting boosts and "that looks pretty" is nice and all, but constructive feedback would probably help more in improving my work. :)