If your iPhone storage space is full and you want your storage to work as completely new, Backup all your Photos to a cloud service. All your Photos would count for a great amount of space and hence you can go for iTunes, which allows you to opt for automatically deleting the Photos once Backup.
Follow easy steps to Automatically Delete all your Photos after Backup :

1. Connect your iPhone to the computer using USB Cable. Open iTunes if it does not open automatically.
2. Go to 'Photos App'. In Photos App, click the ‘Import New’ Blue colour button, in the upper right corner.
3. Next to the ‘Import Selected’ box, you can see a ‘Delete Items’ box, with a check Box next to it. This is the box that allows you to automatically delete the Imported Photos from your iPhone. So hit the check Box to turn the feature ON.

This way your Photos would be Backed to your Mac and automatically be deleted from your iPhone.