HI
Looking to trade my computer in but want to ensure that photos are all backed up. Older photos are not showing in iCloud but want to ensure I don't lose them.
iMac 21.5″, macOS 10.13
First insure your preferences in photos is set.https://www.imore.com/how-to-set-up-icloud-photo-library-mac Do you have other devices synced with iCloud.https://www.imobie.com/icloud/photos-missing-from-icloud-photo-library.htm If in other devices you delete photos ,it can delete the from iCloud.