Delete photos from your iPhone in 8 Steps

Once you’ve imported your photos, you may find yourself wanting to reduce a few (or a few thousand) of the ones you’ve left on your iPhone. Here is how to delete them:
  1. Connect the iPhone.
  2. Open Spotlight and type in “Image Capture.”
  3. Launch Image Capture.
  4. Click on the iPhone icon in the left hand column.
  5. The iPhone’s camera roll appears.
  6. Command-Click each photo you want to delete. Or use shift-click or Command-A to select multiple photos or all.
  7. Click the Delete Icon at the bottom on the Window

The deletion process begins. (If you’re deleting hundreds or thousands of photos, it will take a bit.)

via How do I delete photos from iPhone…: Apple Support Communities.