I post in the void, but most of the contacts I got from replying to self-promo posts others created. Like when an indy publisher or another comic artist with bigger following goes "share your portfolio/comic below" and a bunch of us reply. Sometimes the original poster retweets the replies too, so there's better chances more people get to see what we shared.
There's also general hashtags like "porfolioday" and "selfpromosaturday" when everyone just promotes their art/portfolios in the tag.
I'm still not sure how exactly Twitter works, but it can somewhat detect topics from the post on its own too, without tags. And then it shows up in the topic search results. Hope this helps! My tag there is @martybarok if you want to connect. 