Also, aside from customizing the background for the SC2 login menu, there's also some guides on how to change the picture that show up on the Quick Match page, based on what race option you picked. So for example if you customized the Zerg's background pic to a picture of a pig, the next time you go to Multiplayer and pick Zerg, the background picture will change to that picture you customized. The method to do so is pretty complicated, though.
You can also customize the picture of Starcraft II's in-game's Homepage to the picture you want.
Kudos to teliko for figuring this out and sharing it with the world.
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