The 139 mailbox currently does not support image signatures, but text signatures can be set.

Log in to 139 Mailbox, click "Settings - Account Settings" in the upper right corner, click "New Signature" in "Email Signature", and follow the page prompts to set the signature content.

Or after entering the 139 mailbox, click the triangle under "Signature" at the bottom of the writing page and click "Set Signature". You can also enter the signature settings page, click "Add Signature", enter the title and content of the signature and click "Sure".