Here is the info on changing your signature if you use Yahoo. I copied this information from the Help Menu which is very useful if you know the right question to ask.
How do I add, edit, or delete a signature?
- Sign in to Yahoo! Mail.
- Click the Options menu in the upper-left corner, and select Mail Options from the pull-down list.
- Click Signature at the side.
- From the menu on the right, select a signature option:
- Show a plain text signature: Allows you to create a simple, text-only signature.
- Show a rich text signature: Allows you to choose the font, color, and size of the text in your signature, and also include images.
- Don't use a signature: Select this to turn your signature off. Note that you can delete your signature in a message when you’d rather not send it.
- Next, add or change your signature by typing in the text area.
- To save your changes, close the Options tab (hover over the tab and click the X that appears).
Note: If you create a rich text signature, it will only appear in rich text if you are composing your email in rich text mode. When composing an email, you can switch to rich text mode by clicking the Switch to rich text below the email's subject line.