It appears that the language string
APP_EVENT_PHOTOS_EMAILS_LIKE_PROFILE_PICTURE_ITEM_SUBJECT is missing from en-GB.com_easysocial.ini
I also noticed a couple of others strings that should be changed for proper reading in English. The word 'like', should be 'likes' or 'liked' in the following lines:
APP_EVENT_PHOTOS_EMAILS_LIKE_PHOTO_ITEM_CONTENT="This is a notification to inform you that %1$s like your photo."
APP_EVENT_PHOTOS_EMAILS_LIKE_PHOTO_INVOLVED_CONTENT="This is a notification to inform you that %1$s like your photo."
Just thought that I'd let you know so they can be added or changed in an upcoming release (I've already made the changes to my site).