Social media manager here, and I've some thoughts...
I'm not wholly comfortable promoting on Instagram, not if your comic doesn't have a large following or isn't known in some capacity. This is because you might get more eyeballs on your post on Instagram but it might not translate to views on Webtoons/Tapas or even new readers.
It's very hard to convert the average IG user to being active readers.
However, some comics (like Lore Olympus) have used promotions on IG to some success (I don't have access to their analytics, so I'm not sure how useful promotions are to the comic). What they did however was use carousels to tell a mini story that would end in a cliffhanger, prompting people to read the comic to see what happens next. Kind of like a mini-trailer or commercial.
If you're keen on using promotions, some things to consider:
1) Where will the comic land? Is it right at the latest episode or will readers be able to read from the beginning?
2) Who's your audience? It's best to target people who are potential readers instead of casting a huge wide net. Think about the kind of people who might be interested in your comic and make the promotional material appealing to them.
3) Is the comic accessible? Do readers need to sign up to read the comic or can they read everything for free?
I'd personally recommend using the appropriate hashtags on Instagram (#YourComic'sName+Comic, #Webtoons, #WebcomicsOnInstagram #WebcomicArtist, etc.)
The best bang for your buck would be to participate in art/comic shares on places like Twitter/IG when it goes around. Promo-for-promo stuff is pretty common and helps out in people discovering your work!