iPhoto - how to add text to a picture
iPhoto - How can I add text to an iPhoto picture?
MacBook Pro (15-inch Late 2011), OS X El Capitan (10.11.3)
Are you asking about iphoto on a Mac or iPhoto on an iPhone? This forum is for iPhoto on an iPhone, iPad, iPod touch.
In iPhoto on a Mac is no tool to write directly onto a photo. You would have to use an external editor, for example Preview can write onto a photo.
If you want to add a written caption to a photo in Photos for Mac and have updated to macOS 10.12.2, you can use the Markup photo editing extension directly from Photos's Edit panel. You do not need the detour to Preview any longer.
Select the photo and open the Edit pane. Click "Extensions", select "Markup" or "more", if "Markup" does not show and add it.
In the Markup panel add a textfield and select a text color and size. Drag the text field to the place you want and adjust the size.
Type your text into the textfield, then click "Save changes", and you will see the annotated photo in Photos.