Heres a tutorial on how to add your signature to Iphones, this includes,
Images and text formatting.
Ok the first thing is send yourself a blank email with just the signature you want,
When this is received open it up on your phone, tap and hold then Select All, then Copy.
Once copied head to Settings>Mail contacts and calendars, then the signature section.
Tap and hold select paste, Once the signatures in there shake your phone and select Undo when it asks to Undo Change Attributes
That should put your full signature into the iphone with everything you need even text its font colour etc etc.