Deleting photos in Photo on mac, while retaining them in iCloud storage

Why?

Do not rely on Icloud as a backup. As markwmsn points out, if at some point in the future you re-connect the mac library to I cloud it will sync, and delete all the same images in I cloud. In any case they should never be stored in only one place if you don't want to lose them in future.

If you are short of space on your mac, better to get an external drive and move your photos library to there.

If I uncheck "System Preferences > Internet Accounts > iCloud > Photos" on my mac, can I delete photos in my Photos app but still retain them in iCloud or will it delete everywhere?

It is working for some time, but it will be highly inconvenient. You can no longer work with the photos properly, if you are only seeing them at www.icloud.com. There is no search function in the web version of Photos, you cannot create albums, you cannot adjust the photos or view the metadata.

At least make a copy of your Photos library on an external volume, before you start deleting the photos on your Mac. And make sure you have a backup of your only local copy of your full version of the library.

If you disconnect your Mac from your iCloud Photos library and keep it disconnected, then deletions, additions, and edits will no longer propagate to or from your Mac through that channel.