There’s no simple obvious way to delete all the photos off your iPhone. 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 are two ways to do it.
1. Do a blank sync with iPhoto. Requires a computer but is fast.
Plug your phone to your mac via USB then in iTunes go to ‘Photos’ and select ‘Sync photos from iPhoto’, choose ‘selected photos’ (not all photos) but don’t select any photos or photo albums. When you sync it will erase all photos from your iPhone.
2. Delete them via ‘share photos’ on iPhone. Doesn’t require a computer but it takes a while.
From you iPhone go to the camera, go to the camera roll so you can see all the photos on your iphone, click the ‘share button’ press each photo you’d like to delete, then press the delete button instead of the share button to delete them all.