I want to delete photos from icloud but keep them on my imac.

Syncing with iCloud Photos is keeping the local libraries on your devices identical to the central storage in iCloud.

If you want to keep the photos on your Mac, when you delete them from iCloud, the photos on your Mac must not be in an iCloud Photos Library.

You have two options:

1. Duplicate the Photos Library on your Mac and save the copy of your current Photos Library somewhere on your Mac or an external volume. The copy will not be syncing with iCloud, as only one library (the System Photos Library) can be syncing with iCloud Photos. You just have to ensure that the copy of your library is complete. Before you duplicate the library turn off "Optimise Mac Storage" in iPhotos > Preferences > Cloud and wait for all photos to download back from iCloud to your Mac. Only then drag the Photos Library to an external volume to create a copy.
2. Or stop using iCloud Photos on your Mac completely. Turn off "Optimise Mac Storage" in iPhotos > Preferences > Cloud and wait for all photos to download back from iCloud to your Mac. Then turn off "iCloud Photos" in iPhotos > Preferences > Cloud. Now you can delete photos from iCloud without deleting them from your Mac.

I am using the first method - I am only keeping a small Photos Library in iCloud, with only the selected photos that I want on all devices and larger archive Photos Libraries with all my photos on an external volume. This volume is regularly backed up by cloning it to a backup drive with Carbon Copy Cloner.

Set up photo library and its preferences in Mac Set up and use iCloud Photos - Apple Support

Change preferences in Photos on Mac - Apple Support

System Photo Library overview for Photos on Mac - Apple Support

First archive the photos Archive or make copies of the information you store in iCloud - Apple Support

Or photos can also be exported Export photos, videos and slideshows from Photos on Mac - Apple Support to an external hard drive connected to Mac Use external storage devices with Mac - Apple Support or in finder location .

From Mac photo applicarion select photos Select and deselect photos in Photos on Mac - Apple Support

 If photos are in series , hold the shift key click on rest of photos to get selected ( the first photo and last photo will be selected , select the photos in small numbers ) .

Right click on any one of them and click on delete .

See the article Delete photos or recover deleted ones in Photos on Mac - Apple Support