How can I backup my photos in my iCloud?

Hello, go to setting> your name (Apple ID)>iCloud and toggle on photos

If you're already syncing your pictures using iCloud Photos, they will not be backed up either on iCloud or in a backup on your computer. If you want to create a copy of those photos, you can use this procedure:--> Archive or make copies of the information you store in iCloud - Apple Support

If the're not synced, then they will be backed up (if you have enough space on your iCloud).

Regards

Giulio