Photos to Save

When you get your new phone you will transfer your data from old phonecto new.

However if you lose or break your phone you will lose your photos that are not in iCloud photos

You should back up regularly

Backup

How to back up your iPhone, iPad, and iPod touch – Apple Support

See the transfer instructions in:  Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Are you using iCloud photos? If so, the photos you are seeing must have been synced to the iPhone from the computer via iTunes. Those photos should still be on the computer that you synced from. Just to be sure they are synced photos, tap on one of the photos and see if you can see the trash can available for you to delete it. If a synced photo, the trash icon is not available.