The text or picture at the top of the sign-in box is a sign-in seal. It's a secret message or image chosen to help protect people on that computer from phishing -- a scam that tries to steal passwords or personal information by spoofing a legitimate web site.
If you didn't set up a sign-in seal, someone who also uses this computer has probably created it. If you are sure that no one else set up this sign-in seal, you could be on a fraudulent website -- one that looks like a Yahoo! page but is designed to fool you into revealing your Yahoo! ID and password.
If you're at all uncertain, do not sign in. Here's the safest method to sign in to Yahoo!: