Overwatch Origin skins from loot boxes
From an interview with Jeff Kaplan,
Similarly, while there's currently no “purchase-only” content in the game beyond the Origins Edition and Widowmaker preorder skins [those won't be unlockable for credits because, he said, “We want those to feel special and exclusive”], it's simply too early in the process to commit to it one way or the other.
The answer is you cannot unlock them with credits.
You can't get them from loot boxes, they are exclusive to the Origins edition.