The sheer fact that someone ACTUALLY found hidden images in the CSS file.... amazes me. Someone has WAY more free time than I do.
Though investigating further, it appears the hidden image is possibly an homage to Michael Dashow, Designer on Diablo 2.
Also of note, Blackthorne trademark had been updated by Blizzard.
MYSTARY!
I wish Obsidian could do some cool thing like this on one of their next MYSTARY games in sekret development....