easy way to add a caption on a picture
I am having a hard time adding a text to an iPhoto picture
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Are you using iPhoto or Photos?
Do you want to add the text as a graphics overlay to the picture? Or do you want to add a title to the picture as metadata in Photos or iPhoto?
To write text onto a photo in Photos for Mac you can use the Markup extension. It wll work in PHotos 4 and Photos 5. See this Help page:
Use Markup to write or draw on a photo in Photos on Mac - Apple Support