You can set a Global Signature in VPOP3 by going to Settings → Global Signature
The Global Signature option lets you tell VPOP3 to add a standard piece of text or HTML to the bottom of all outgoing messages.
You can create just a plain text, or HTML signature, or both. If you create just one format, then the other format will be generated automatically if necessary. However, it is generally best to create both formats, so that you can be specific in what is sent.
Note that just specifying an HTML signature will not cause that to be added to all outgoing messages. If a user sends a plain text message, VPOP3 will convert the HTML signature to plain text (eg by stripping HTML tags) and add the converted version to the outgoing message.
When you create an HTML signature, note that this will NOT embed other content into the message, so if you include an image in the signature, the image will not be embedded in the outgoing email. The included image should be a link to an image on your website (or other website). Then, when the recipient receives the message, their email client will retrieve the image from your website. Using linking rather than embedding is generally considered better, because it means that the image is not sent on all emails, and if the recipient is using a mobile phone, for instance, to view the email, that will usually have an option not to download the image to save bandwidth charges.