Photos and iCloud cannot see photos on iCloud safe to delete?
If you enable iCloud Photo Library on your Mac, and your photos are on the Photos app, you can set it up to save originals on the cloud and automatically save space by keeping a lower resolution copy on your Mac.
To do this, go to Preferences > iCloud, then select "Optimize". 

This will also allow you to view your entire collection on iCloud.com if you wish.

Note that you have no saying on when the app starts "saving space" on your behalf, and that if you delete a photo on your Mac it will be deleted on iCloud too; it is more a synchronization service than a storage on the cloud service, with the advantage of the space optimization.

Also, there is no Mac iCloud backup service to my knowledge; you can only backup to Time Machine —or some other backup service of your preference, of course. You may have some of your documents synchronized to iCloud but if you delete them locally they will also be deleted there.
Time Machine erases old backups when it runs out of space, so it won't work as a long term storage solution either, unless you have lots of space available —so it does not need to delete your old backups in order to make space for the new ones.