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.