Q: how can i delete photo events from my iMac on my iPhone

If they were synced to your phone via iTunes you will need to sync again but use the instructions quoted below from Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support

Delete synced photos

1. In iTunes, locate and click on your device.
2. Click Photos.
3. Choose "Selected albums" and deselect the albums or collections you want to delete.
   To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
4. Click Apply.

Learn how to delete photos and video synced from iTunes when you can't access the originals.