I want my photos on iCloud and not on the hard drive of my new iMac 24. How do get them off the hard drive but keep them on iCloud?

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

In Photos>Settings choose iCloud and "Optimize Mac Storage." This will keep smaller copies on your computer so that you can see them quickly, but the full sized images will be stored in iCloud.com where you can also see them in a browser. It will take some days, probably, for all the pictures in your Photos Library to upload to iCloud. The library on your Mac will get smaller as you need more space. If you want to edit or print your pictures, Photos will download the full sized images without you really noticing.

There's more detailed information here:

Set up and use iCloud Photos - Apple Support