Do it on social media, maybe, but whatever you do, DO NOT put that stuff as episodes at the start of your comic. The absolute last thing you want is for people to click on your comic and have to wade through a bunch of concept art to start the story. Getting the story rolling and showing the great looking pages as soon as the reader starts are really key to making a splash with your Tapas launch.
All that said, just posting concept art can be a risk if the concept work isn't polished. If you have an established audience who are fans of your more finished work, they might be interested in seeing sketches as extra content, but for people who aren't familiar, showing them scruffy sketches might put them off, so sharing more finished looking promo artwork, or panels from the upcoming comic might be better.
As a guideline, I'd say posting stuff in advance is less about familiarity and more about anticipation. So the stuff you post needs to be really cool to make it work. Like it needs to be imagery that'll get people excited for what your comic is going to be about. This might be because it's a spectacular piece of art, a really good character design, or art that promises certain appealing tropes in your series or depiction of underrepresented groups.
But also, as Llama said, be very wary of spending all your energy posting promos rather than just focusing on making really good comic pages! Making the comic good and making a great cover, logo and banner is probably the best thing to focus your energy on first and foremost.