Here is one simple method I got from here: http://www.mactrast.com/2012/12/how-to-remove-all-photos-from-an-iphone-at-once/
- Connect the iPhone to the computer to the Mac’s USB port using your iPhone sync/charging cable.
- Launch Image Capture. You’ll find this in the Applications Folder.
- Make sure the iPhone is selected in the device list on the left side of the screen, and Press (Command Key + A) to Select All the photos.
- Click the red Delete button, at bottom of screen (red circle with slash through it - as shown on link page), to delete all of the photos.
- Confirm deletion when asked.
- Wait for the deletion process to finish. (If you have Gigs of photos on your iPhone, this can take awhile.)