Locating missing photos from a lost iPhone

If you had iCloud Photos turned on before you lost the phone, use any internet browser and log into www.icloud.com and go to the Photos app there. You can also turn on iCloud Photos on a replacement phone or other devices and the photos will automatically sync with it. See: Set up and use iCloud Photos - Apple Support

If you didn't have iCloud Photos turned on but do have a backup to iCloud or to a computer, you can use the backup to setup a replacement phone. See: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Did you ever back them up to a computer or special backup device? You can get them there.

Did you have iCloud Photos enabled and storage available in iCloud? You can get them there.

Did you have some other cloud photo storage or syncing system connected to the phone? You can get them there.

If the answer to all three is no, and you cannot find your phone, then your images are also lost.