fail maker was designed to write fail! or pwn! on your own pictures but it also allows you to create you own message, there is also stick it which is cool it allows you to be very creative. but it's 99cents.
the 2 screenshots with the grocery list on them are from stick it
and the one that says fail on it us obviously from fail maker
IfFoundPlus is a great app to write on pics for your home screen or other uses. Can add your phone number to your home screen picture. Simple but essential.
I know penultimate will allow you to handwrite on a photo, but not type. I believe notability is the same way with handwriting, but it may also allow you to type. I am sure there are others but these are the ones I know of off hand. Of course these are iPad only. I am not sure about iPhone apps.
Photogene is of course a great app. But if you really are interested in writing on photos, check out ScribeIt! You can choose your color, font (there are tons), shadow, alpha & font size. It's quite powerful. Way more powerful that Photogene. I know, I have both (including the newer Photogene).