This is another app that I like quite a bit. They also have a free and pro version, but the free one does a pretty good job. You can add thought bubbles, speech bubbles and text shapes to pictures and also create interesting photo collages, which is one of it’s unique features. You can also add smileys and clipart to your photos.
If you want to simply add plain text, you can do that too. It also has a nice interface and a lot of controls for color, rotation, effects, etc.
"iPhone Tip- Apps to Write on Pictures-Photo Notes"
Reference : online-tech-tips.com