What's a Yahoo! sign-in seal and why do you need one? "A sign-in seal is a secret message or image that you select to help protect your account from fraud such as password phishing. The sign-in seal you designated is only saved on computer you created it on – therefore if you log in using a different computer, you'll not see sign-in seal. If you use more than one computer or browser, you can choose to create a sign-in seal for each. If you don't see your sign-in seal on sign-in page, webpage you've opened is likely a fake or "spoofed" Yahoo! page. If this happens, you should report the incident to Yahoo! customer care immediately!
Another important tip: do NOT create a sign-in seal on a computer you share with strangers such as those in libraries, Internet cafes, and other public hot spots.