How to delete photos from iPhone 5 without removing already transferred photos from iMac?
Were you transferring photos from your iPhone to your Mac using Photo Stream? If you were, no matter what happens to your photos on your iPhone, the transferred photos will always be on your Mac.

Because the Mac would most definetly have more storage space than an iOS device, Photo Stream photos are automatically downloaded and permanently saved to your Mac's iPhoto or Aperture library; the 1000 photo/30 day limit doesn't apply for OS X or Windows unless you clear the photos and start fresh.

If you moved or copied your photos from your Photo Stream to somewhere else on your Mac, they are still safe.