iPad Mail doesn't currently support rich-text signatures so there's no way to include your miiCard card image.
The following is an example of how you can link to your miiCard in your iPad Mail signature.
t: 01234 567 890
Identity assured by miiCard: http://miiCard.me/johndoe
To change your signature:
- Open Settings from your home screen.
- Choose "Mail, Contacts, Calendars".
- Choose "Signature".
- Enter your new signature, complete with a link to your miiCard. The default signature reads "Sent from my iPhone" or iPad, or iPod Touch. You may wish to keep something like this as it tells your recipients that you typed your email on a mobile device which may explain the odd typo and rushed nature that we all have on these devices.