How to retrieve photos from a lost iPhone?

Depends on if you were using iCloud Photos or had an iCloud Backup.

• With iCloud Photos, they are available by signing into iCloud online where you can download them.
• If not using iCloud Photos, then photos are saved in the iCloud Backup by default, and when you set up a device from that iCloud backup, all photos will be restored to the device. If not using the iCloud backup, then your photos are lost.