The official Dishonored Twitter account tweeted the image above along with the cryptic, "News tomorrow", opening the Internet and games writers everywhere to speculate that Bethesda and Arkane will be announcing new DLC for Dishonored tomorrow.
The image appears to be from the opening scene of Dishonored, when Corvo meets with the Empress and her daughter after his trip seeking help from the rat plague, just before Daud's assassins kill the Empress and kidnap her daughter right before Corvo's eyes. However, the angle of this image seems to be from the point of view from the assassins, leading many (myself included) to believe this DLC could be from the perspective of one of Daud's assassins. If so, that could be interesting indeed.
But I suppose we'll have to wait until tomorrow for any official announcement.