Hi elsaliz,
Thanks for coming to Apple Support Communities for help with storage space and Photos on your Mac.
It sounds like you are using iCloud Photos and if you do delete a photo, then that photo will be deleted everywhere you use iCloud Photos. Please see: Set up and use iCloud Photos
But it sounds like you may have the option to Download Originals to this Mac enabled, which would download the full sized photos to your computer for storage. You can save space by choosing Optimize Mac Storage in your iCloud Preferences as outlined here:
Change preferences in Photos on Mac
If you need to free up storage on your Mac, this article can help: How to free up storage space on your Mac.
Have a fantastic day!