iOS 6 supports HTML signatures in Mail. This is an advanced tip that requires being rather familiar with HTML.
- Create your signature in an HTML file. Images will need to be published on a web server and referenced in the HTML markup.
- Get the file onto your device (Dropbox works well).
- Load the file so it displays formatted text, not code.
- Settings app > Mail, Contacts, Calendars > Signature.
Mail will keep the HTML formatting including fonts, images, and links.