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:
- Connect the iPhone.
- Open Spotlight and type in “Image Capture.”
- Launch Image Capture.
- Click on the iPhone icon in the left hand column.
- The iPhone’s camera roll appears.
- Command-Click each photo you want to delete. Or use shift-click or Command-A to select multiple photos or all.
- 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.
Data Enhanced 