Photos on iCloud

It seems like you may have iCloud photos set to optimize space used. In Photos > preferences > iCloud check the download originals to this Mac. That will keep your original file on your computer and in the cloud.

How are planning to "put all the photos from my iMac on the iCloud"ß With iCloud Photos or with iCloud Drive?

Both cloud services - iCloud Drive and iCloud Photos will mirror the photos on your Mac. If you delete an item on your Mac either from iCloud Drive or iCloud Photos, they will be deleted from iCloud as well, because iCloud is a syncing services, trying to Mac the items on all synced devices identical to iCloud.

If you only want to remove the photos temporarily from your Mac, you could sync the system Photos library with iCloud Photos, and after all photos have been synced and uploaded to iCloud, move the system photos library to an external drive (to keep a backup copy of the library, just in case). Removing the complete Photos Library will not delete the photos from iCloud. You can later create a new, empty library and sync the photos back to this new library with iCloud Photos. The originals and the edited versions will remain paired, all albums and metadata should survive this, and you should still be able to revert individual adjustments.

While you have no local iCloud Photos Library on your Mac, you can download individual photos from iCloud at www.icloud.com, either the original or the edited version.

If you delete any photos on any device connected to the cloud, it will delete from all devices and the cloud