First off, i just want to say i'm in no way affiliated to the following app - App Store - Email Signature Pro - it's just i purchased this app today and am really really impressed with it and thought i'd like to give it a reccommendation to users here, as the question of email signatures often arises.
Basically, it allows you to create up to 6 separate email profiles or signatures. These include clickable/linked pictures, banners or/and text, all of your own type, size, shape and design, from your own server or web space. And it all works most closely with the standard ios email client.
I have 3 separate businesses to over see, and family and friends. This app lets me make separate footers appropriate for each client or friend group, and saved/used at a single click. Makes life even easier than using a desktop email client. It's not in HD version as of yet (altho they plan a version soon), but as mentioned, works a treat. Early days for me with it, but i've yet to encounter any issues or downsides with it.