So, while modifying the files located inside the "Starcraft II/mods" folder is obviously not a violation of the EULA (so you can load the game without being required to get a stare-down from Jimmy during the loading screen).

I don't know if Blizzard has an official stance on modifying the battle.net background images. There's a pretty comprehensive guide as to how to do so (for both the Jimmy-B-gone and the .net screens) but my question is have any of you reading this used a modified background during a patch without issue?

http://telikostarcraft.com/modifications/customise-background-images/

Personally I'd like to see a medic looking back at me instead of a bunch of battlecruisers...