Okay, figured it out. I just set the Dark Souls profile for both .exes (data and dark souls), and now I've set other profiles that toggle on and off with corresponding games. Awesome.
The bad news is that Steam keeps deleting the custom banners every time it updates. Is there a way to prevent this? Otherwise these custom banners will never stick.
Edit: My custom icons carried over with the latest update, so maybe this has been fixed on Steam's end. I hope so.
I'd like to request some SNES icons I don't see in the banner site.
Super Punch-Out
TMNT: Turtles in Time
Power Rangers
Batman Returns
X-Men Mutant Apocalypse
Whenever you add a new game, exit steam and restart it before you add the picture. It should not disappear anymore. Have no idea what causes it.