Then how does the image consistently display before the password has been provided? No matter what the answer is, I don't see how it could be an anti-phishing feature.
I use fresh incognito tabs constantly. So I guess I'll never see the image, and never know something is amiss.
By I also never click an email link to login unless it's a plain text password reset. I receive authentic looking and topical Dropbox share requests from actual contacts (who have been hacked) trying to phish my Dropbox credentials maybe 4-5 times a year so I'm always on the lookout for it. This is a classic attack. Always check the URL!