How do I take photos off iPhone without taking them off my iMac?

Hi ejay1,

Thanks for using Apple Support Communities! We see that you have a question about removing photos from your iPhone, but without removing them from your Mac or iCloud, and we'd like to help explain.

With any item synced to iCloud, such as the Photos app, removing an item from one device also removes from iCloud and other devices with the same Apple ID using iCloud for that item.

You can do other things, besides using iCloud to sync your photos on your iPhone. Such as:

Turn iCloud off and keep it turned off at Settings > Your name > iCloud > Photos on your iPhone. Photos on your iPhone will be independent from iCloud and your other devices. This means, in order to save any new photos to iCloud, you'll need to AirDrop a photo to another Apple device, which still syncs with iCloud.

How to use AirDrop on your iPhone, iPad, or iPod touch

Or, you could save your photos elsewhere, and then delete from all devices and iCloud. This article has how to archive items you keep in iCloud, such as Photos:

Archive or make copies of the information you store in iCloud

Take care!