Go to yahoo mail site and you will notice in the log in area "sign in seal" in the upper right corner.
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Create a text seal... Type a short, secret message for this computer
21 character max
* Create a unique message that everyone using this computer will recognize—for example, your street address.
* Don't include your ID or password!
Upload an image... Select an image from this computer to use
* Choose a photo that everyone using this computer will recognize.
* It can be JPEG, JPG or GIF and should be under 2MB in size.
* Tip: Use a close-up that will look good when at small size.
Note.If you have two or more personal computer you should create one sign in seal in each computer.