This is a crazy oversight by Apple, but there’s no obvious way to delete all the photos off your iPhone. You can delete them from the iPhone but you have to select each photo one at a time. Digital Cameras usually have a ‘delete all photos’ option but the iPhone doesn’t. If you don’t delete them when you sync them to iPhoto, it’s hard to get rid of them.

Here’s how to do it.

In the utilities folder on your Mac computer you will  find an application called “Image Capture”.  It is made by Apple, and it will allow you to delete all the photos of your iPhone as follows:

1. Plug in your iPhone  to your computer.  If Photos and iTunes open just quit them.
2. Open the ‘Image Capture’ app.
   (It will be in your Applications folder, called ‘Image Capture’)
3. Select all the photos that you want to delete.  
   (To select them all in one go click on the top photo, scroll down to the bottom and then shift – click on the bottom photo to select them all.)
4.  Click on the no smoking icon that is in black. It looks like a circle with a line through it. For some reason Apple thought this was a good symbol to delete instead of using the normal trash symbol,  I have no idea why they would do this.
5. All your photos will be deleted – it’s that easy!
6. 
   Thanks to Dan for this tip.

The problem with Apple’s suggested way of deleting photos.

This is what Apple suggest on their help site:

The problem with this is that you need to do this for every single photo individually. This takes a long time if you have a lot of photos!