Please note, this tutorual was written using the Advanced login interface in @mail, the interface may look different if using the simple interface.
To add a signature, including images to your webmail signature, login to @mail, by visiting http://webmail.YOURDOMAIN/
(Please replace YOURDOMAIN with your actual domain name).
You should then see a window as below:

Once logged in, click the Preferences menu within @mail, and choose Account Preferences
Scroll down to the "Block images in email messages" section

There are 2 options, you can either show all images, in which case you can simply add images to your signature.
If you only wish to show images from known contacts (i.e. people in your address book), you will need to add an entry to your address book for your own email address, including your domain name. You will then be able to add images to your signature.
Scroll down to the signature section, here you can add your signature, including images. To add an image, you simply need to use the HTML <img> tag, please note you will need to use the full domain name to reference any images in your webspace.

Click Save settings to save the signature