If your host is a UNIX/Linux server and they allow cron jobs you can setup a job to run a php file from command line at a certain time.
Another possiblity is if you can modify the code that allows users to add messages/photos that after it writes the data to a database or uploads the photo it sends an email to all the required people notifying them. This would be more real-time and you wouldn't have to worry about your script kicking off to run this.
I thought about creating te script after each message, picture, etc. However, if there are 15 messages created in a day thats 15 messages. I need to compact things to once a day. Within one email there would be 15 links linking each message for that day.