One of the things lacking from iOS's built-in Mail application is HTML signatures. Free web app iPhone Signature Creator, however, helps you create a home screen shortcut to send messages with signatures containing links and images.
It isn't quite as streamlined as built-in support, but it's really easy to pull off. Just head over to the web app, fill out the form with the information you'd like to appear in your signature. You can fill out anything from name, title and company to your phone number, website, logo and even a FaceTime link. Once you hit Create Signature, it will give your a URL that, once accessed on your iPhone, will prompt you to save the page to your home screen. Afterwards you can use this shortcut to create a new message in Mail with your custom signature.
The only trouble I had with the creator was that it didn't seem to like my linking to a photo on the web for my logo. Once I downloaded the JPEG and re-uploaded it to the signature creator though, it worked like charm, as did most everything else. Hit the link to check it out.