March of the Lich King Cards List & Guide
At the end of the first questline, you get a legendary merc card for completing it. I think it's at around the 6th bounty.
I wish I had known that before burning through most of my packs to get the final legendary merc, before starting the quests
OK EDIT - Reading online, it seems like some people are getting portraits even if they are missing legendary mercs, while other people are getting a merc they are missing.
If you are missing a merc then perhaps hold off on claiming the reward until Blizzard clarifies, since it may be a bug that could be patched.
can vouch i have all the legendarys except the new ones completed the quest got a rag portriat you also get one for task 11 the last guardian