Are the photos on my iPhone saved on the cloud? If I delete from my phone are they deleted from the cloud

Whether or not they are on the cloud depends on what settings you have set on your phone. If you have turned on Settings > iCloud > Photos > iCloud Photo Library then they should be on iCloud. You can verify that by logging in to iCloud.com and looking at the Photos app there. However if you have that turned on, if you delete from the phone they will be deleted from iCloud and any other device connected to iCloud Photo Library. The most you can do to reduce the storage required on your phone if you are using iCloud Photo Library is to turn on Optimize iPhone Storage on that same Settings screen.

If you are not using iCloud Photo Library you can import the photos to your iMac and then delete them from the phone: Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC - Apple Support