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Photos doesn't allow you to add texts on the photos. To add text on the photos, you have to use Preview application. Click and drag out the photos from Photos app to desktop and this will save a copy of photo in desktop. Then open that photo in Preview. To do that, double click on it or right click (control + click) on the photo select Open With and select Preview.
Then add the text in Preview. Follow the instructions in here --> http://osxdaily.com/2013/05/16/add-text-to-photos-mac-preview/
Then save or export the photo from File menu (save it to desktop). After saving the photo, click and drag that photos to Photos icon on dock and this will import to Photos. Now you can put it on your calendar.
If you have any difficulty on following the steps above, please let me know and I will continue to help you until you fix this issue.