It seems you have an issue about lack of available storage space on your iPhone. In this situation, you should activate “Optimise iPhone Storage” option.
After activating it will safely store all the pictures and videos in iCloud storage. All the iPhone pictures and videos would be resized to a lower resolution.
You will still be able to see them but if you want to edit or transfer them, an Internet connexion must be needed. It will download the original copy from iCloud storage.
- Open your iPhone and go to the Settings app
- Scroll down menu and select on “Photos” (for iOS 11) or “Photos & camera” (for iOS 10 and earlier)
- Now turn on “Optimise iPhone Storage” option
Now your phone just needs an Internet connection and it will automatically upload all the new pictures on iCloud.