The show demonstrated that Discord simply changed their personalities on the spot. The artist here, is demonstrating that each individual pony began seeing a duplicate of themselves as soon as they were Discorded, that verbally taunted them and, basically, 'trolled' them into being disharmonized. But, only the pony of whom the duplicate belongs can see it and hear it, I assume. Effectively, Discord gave each of them their own manifestation of negativity that only they could see. It's rather clever, and very interesting. Hope that helped.
Yeah, it's not so easy since it isn't fully colored yet, but the two tricks are looking at the manes to see if they match, and looking at the cutie mark on the duplicate pony: Each of the duplicates has a Discord face symbol.
Discord, you adorably evil jerk! What did you do?? Anyway, I feel so bad for two characters from this pic. One is Fluttershy (those eyes!!!), and the other one is Discorded Pinkie, she looks so miserable! Pinkie must think the same, judginf from her face