There are many notepad apps that let you insert a picture you have taken as the background notepad paper, write on top, then save as a new image. These generally offer features you need for creating notes on your iPhone but overkill for just adding a quick line of text to an image.
The free LabelBox app opens to a diagram to help you get started quickly.
When you start, you are presented with a row of text tapes you can use on your image. Swiping across the screen to show more options. the last four can only be accessed if you pay via in-app purchase. Tapping a ribbon will result in that one being used when you swipe to add on the photo. Multiple types of text ribbons can be used on a single iPhone photo.
Across the bottom of the LabelBox screen, left to right: Camera allows you to take a picture or use one from your photo album, next icon allows you to save a copy to share, and the last two are for using the app’s online photo storage service.
When back on LabelBox’s image view, just swipe across the screen to create a tape to add text to. Getting back to change the text tape being used, tap the small arrow icon at the bottom/middle of the screen.
Tapping on a text box brings up the keyboard. Double tapping a text stripe will remove it. I have yet to find a way to move a text box, luckily it is fast to delete and create a new one. I’m looking forward to being able to move the LabelBox text box though, and maybe be able to twist it to get it just right.
Throughout LabelBox, you will be given the chance to sign up and use the Steply photo sharing service.