Blizzard have revealed the card back design you can receive by reaching Rank 20 or above in the Hearthstone ranked ladder for January 2015.
If you want it make sure you reach Rank 20 in the Ranked Ladder this month, and that does require you to log on at least once this month, even if your rank last month has given you enough stars to boost you to 20 instantly. Can't claim the bonus stars if you don't log on. Make sure you get a few higher on the ladder too this month so that you've got a boost next month too.
Happy gaming my fellow tavern-dwellers!