how to delete synch photos from iMac to iPhone

Elaborating on what muguy said, you can repeat the sync from a Mac, selecting a different group of photos/videos, including none at all (an empty folder). Photos and videos not selected to sync will be removed from the phone.

An alternative way to blow away all the iTunes/Finder-synced photos and videos is by enabling iCloud Photos briefly. (The two sync methods cannot coexist on the same device.) If you have an existing iCloud Photos library, some of it will start to download to the phone, which will eat up storage and bandwidth, so this may not be a preferable alternative if you have no other reason to use iCloud Photos on the phone.

If you do not use iCloud Photos, connect your device to your Mac and navigate to the device in finder. You can then choose what albums sync