Why do you want to save your Photos in iCloud? Do you want to save storage on your Mac or do you want to sync the photos to other devices?
The easiest way to store your photos in iCloud is to drag the folders with photos from your Downloads folder to iCloud Drive. You use the Finder to manage files on iCloud Drive, as described here: Work with folders and files in iCloud Drive – Apple Support (UK)
As long as you do not enable "Optimize Mac Storage" to save storage on your Mac, your regular Time Machine backup of your Mac will include the items on iCloud Drive and you will have a backup of your photos. But if you want to save storage on your Mac by using iCloud, you will have to back up the photos manually somewhere by copying them regularly to an external drive.