Over on reddit, Hippalus dropped off some info regarding when the next ( and final ) batch of Holiday Nashor icons will be distributed and finally revealing how players can earn the Shopkeeper icon!
"The Snowdown behavior data is being analyzed by the Player Behavior team and the final batch of Santa Baron icons will be granted when that is complete (next week at the latest). They are more concerned with identifying the right players than with getting them out quickly.
The Shopkeeper icon will then be granted to anyone who earned at least 4 of the other icons (for playing a premade, gifting RP, gifting a champion, gifting a skin, and/or behavior).
Thank you for asking politely!"
While the 2012 Snowdown Showdown event is now OVER ( meaning the rest of the icons are no longer obtainable ), you can check out this post for more info on how each icon was obtained.