I've looked into this - most of them are holiday unlocks. Play on certain holidays & complete the daily challenge to get them. I earned 1 a few weeks ago for Halloween (through some random server glitch). It gave me a really cool picture of the Scarecrow
from Arkham Asylum & a flaming Jack O' Lantern emblem. Other holidays that should be viable are Thanksgiving, Christmas, Easter, & St. Patrick's Day.