Email notifications are sent when you receive a comment on one of your pictures. But in order to reduce the amount of emails you receive, they are only sent if you have not been on the site for a day. And no more notifications are sent until you return to the site.
So if you are on the site every day, you won't receive any email notifications.
Don't forget to check your spam folder if you think you should have received a notification. Please allow emails from firstname.lastname@example.org through your spam filter.