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Unforunately you can't. iPhoto doesn't allow you to add Text. However, you can open the photo in Preview on your Mac and add text. Just click and drag the photo to your Desktop, then double click it, and it will open in Preview.In Preview, click on Tools menu, select Annotate >Add Text. Then save it from File menu. Then click and drag the photo from desktop to the iPhoto icon on the dock.
If you have any difficulty on following the steps above, please let me know and I will walk you through it.
There is a way to change the title of photo in iPhoto itself, but it is not similar to adding text to the photo. See here --> http://www.dummies.com/how-to/content/ilife-11-how-to-edit-titles-of-iphoto-pictures.html
To add text on photos, you need to use Preview applicaton. See this video --> https://www.youtube.com/watch?v=Sp6x_vKPQE8