How do I back up photos in iCloud?

Backup or sync?

To sync, see instructions in Set up and use iCloud Photos - Apple Support

To backup, see: How to back up your iPhone, iPad, and iPod touch - Apple Support

Note that if you sync with iCloud Photos, you can view your photos at www.icloud.com (after the original sync completes) and that your photos will automatically sync to any of your other devices where you have turned on iCloud Photos. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. Syncing occurs dynamically after any change.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.