The only signature you can insert is a typed one.
If you want a proper signed one, you need to draw it first of course. Apps like 'autograph' allow you to do this, but it's an image you're creating, so the iPad treats it as an attachment.
The only way to add a drawn graphic or sketched signature to your email footers is to add an image with HTML code into your settings. But you can't do this on iPads or iPhones email app. You need to do this via the web client or desk top client.
I've a drawn signature embedded into my emails, but I set this up directly via yahoo. Again, it's not something you can set up in iPad. Any signiture you draw will only come downloaded with the email as an attachment.
You can set a drawn signature up in your main email client and your ad will happily recognise it, but you can't set it up in your iPad itself.