I've read of people who assumed Han Ji Pyung is not the main lead based on promotional material, but I honestly can't see why people assumed Nam Do San is the main lead, so let's have a detailed look at the posters together, shall we?
THE MAIN POSTER
I think it actually suggest a more common ground for HJP and SDM (which is kind of shown with their similar views and personalities in the show) and also that they are going in in the same direction since both their upper bodies and their legs suggest they're going on the left upper corner. On the other hand IJ and NDS go on the right and down (opposite to HJP and SDM). The heart shape may be just a coincidence. NDS is the ONLY one NOT smiling.
"COUPLE" POSTER
I like how their clothes colour are matching, thought they are opposite (blue top, light bottom and vice-versa) . But again their body language is odd, like on the previous poster DS is not really smiling, while DM is and most importantly, they are facing opposite directions with their whole bodies (again), not facing or looking at each other. It's a bit similar to the pose the main leads from Kill Me Heal Me had on that drama's poster, but in that case they were each other's support, like a pillar, while here the leads look overall distant.
SECOND POSTER If you look at their upper bodies you see HJP and NDS and IJ hiding their hands, like hiding a secret (letters? their true feeling?) . SDL has folded arms while leaning towards NDS (who tho leans in the opposite, IJ's direction). In body language it could be considered like a self-hug or sign of shyness, self-soothe. Although facing each others directions on the upper part, NDS's and IJ's legs are actually going outside, like they don't want to stand next to each other , but still they both have white shoes. ABOUT THAT. Look at SDM's and HJP's shoes... they're both black and it looks like their right legs go towards each other. Not only that, there is also an optical illusuion of SDM's foot being HJP's foot pillar , there to support his leg (like a hand blocking Pisa's tower from falling in tourists' photos).
LET ME KNOW WHAT AR YOUR THOUGHTS. WHAT ARE OTHER DETAILS THAT YOU NOTICED?