When you create an email signature in Yahoo Mail, it is added to all of your outgoing emails. The rich text formatting toolbar does not include an option for images. However, just as you can insert images into the body of a Yahoo Mail message, you can also add inline images to your signature.

Paste an image into the signature box to include it in your signature.

Select the gear in the top right corner of Yahoo Mail, then select More settings.

Select the mailboxes on the left.