How do I get the photos from my iMac to iCloud?

See the following Apple Support article for information on iCloud Photos: Set up and use iCloud Photos - Apple Support

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.

If you already have set up iCloud Photos, why do you think, the photos are not in iCloud? Could you give us a bit more background information?

Have you checked on your iCloud webpage at www.icloud.com if the the photos are appearing in iCloud?

The first syncing with iCloud will take several days, even weeks, if your iCloud Photos library is large. Be patient and do not change the iCloud Settings, while the upload is in progress.

Once the iCloud syncing is done, you will be seeing your photos both in iCloud and on your Mac. Use the "Optimize Mac Storage", if you want to save storage on your Mac, then iCloud will manage the local storage on your Mac and free storage, if you need it.

If photos in Mac are on the desktop screen just drag and drop them over system photo library of photo application , they will merge in www.icloud.com