Select the thumbnails of the pictures you want to assign titles to. Choose Photos→Batch Change. A tiny dialog box appears with the Set and To pop-up menus. Select Title from the Set pop-up menu, and choose an option for the Title from the To pop-up menu.
For More Information Please Refer: https://www.dummies.com/computers/macs/ilife-11-how-to-edit-titles-of-iphoto-pictures/