Free up iPhone storage by deleting photos

Store your photos and videos in iCloud. When you use this all high resolution pics are stored in icloud and you will free up a lot of phone memory. Make sure you let it back up so that you dont lose any pics. It can take hours to a day, depending on the internet connection.

Your photos and videos are stored on your device in their original, high-resolution version. This means that they use a lot of space on your iPhone, iPad, and iPod touch. With iCloud Photos, you can make more space available on your device and have your entire collection, everywhere you go.

Get started with iCloud Photos:

1. Tap Settings > [your name] > iCloud > Photos.
2. Turn on iCloud Photos.
3. Select Optimize iPhone Storage to save space on your device.
4. 

With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.

khajoti is correct, but note that you cannot actually delete photos from your phone if you sync to iCloud, because deleting them from your phone will also delete them from iCloud. Optimize Storage, as recommended, will reduce their size on the phone to 25% of the original size.