back up photos to iCloud

Do you want to store your photos in iCloud or do you want a backup? I am not sure, what you want, a safety copy of your photos to keep the photos save or to get photos off your computer. If you move your photos to iCloud, you will still need a backup copy of your photos on an external drive, so you can recover photos you accidentally delete.

iCloud photos is neither a full backup nor a full off-site storage. What you will get, when you enable iCloud Photos will be an easy way to keep the photos library identical and in sync across all your Apple devices. The photos will be stored in iCloud and mirrored on all devices, any changes to your photos will update across all devices.

• Even with iCloud Photo you will still have only one copy of your photos - the main copy in iCloud and you still have to set up a rigid backup scheme to keep backup copies, for example use Time Machine on your Mac. But you will have the bonus of an off-site storage. If your Mac or iPhone gets damaged, lost, or stolen, you can download your photos again from iCloud. But if you accidentally delete photos permanently, the will be deleted from iCloud and all devices as well and cannot be recovered from iCloud.
• iCloud can help you to save storage on your Mac. You can save storage with iCloud Photos, if you enable "Optimize Mac Storage". Photos will remove some of the original image files from your Mac and keep only optimized, smaller thumbnails for browsing. This is fully automatic. If you need free storage, Photos will remove automatically more photos from your Mac. But it will use as much storage for your photos as it can, so your most recently used photos will not be optimized. Fully optimized, your library can be reduced to 10% to 20% of its original size. The draw-back will be, that you will need always an internet connection to work with your photos. You must not delete any photo or video on your own, if you want to keep this item. Photos will optimize more and more photos for you. But it will be more difficult to make a backup of your Photos. You cannot control, which photos will be stored locally or will be optimized. Time Machine cannot backup optimized photos. You will have to keep a copy of your Photos Library, that is not optimized on an external drive. And make backup copies of this library with Time machine or a cloning application.

See: Keep your photos safely stored and up to date on all of your devices - Apple Support