The testcase loads the same image as an <img> and as a background image of a <div>. If the image doesn't finish loading before the page is painted for the first time (I've added junk to the URL to avoid caching), you'll see it loading in the <img> but never updating the background of the <div>.
Created attachment 6082 [details] Testcase
Regression occurred somewhere between r12443 and r12450.
Looks like RenderObject::setImage just needs to be renamed to RenderObject::imageChanged.
Verified in r12476 nightly
Removing Regression keyword from bugs already fixed.