how to back up Mac photos to my icloud

There is no iCloud backup for photos, but you can store your photos in iCloud, if you enable iCloud Photos. However, iCloud Photos is not meant to be a backup. It is a syncing service keeping the Photos Libraries on all devices identical. And as an added bonus, we are having an off-site storage of our photos. To have a backup copy as well, you could export the photos to a folder on iCloud Drive. You can store individual photos there, but not a copy of the Photos Library.