I had a similar issue.
On iOS 8+ if you sync with iCloud, your Mac or other external source than the trick with Image Capture will not work. I had over 2,000 photos on my brand new iPhone 6+ sync'd with my Mac and in Image Capture there with exactly zero images that showed up. I unchecked the sync option in iTunes for photos, tried to sync with iCloud, etc. The phone was not letting me manually deleted the images (which would have taken days anyhow). Nothing worked, the photos were stuck.
The WORKING solution was to create an empty folder (I made one named TEMP on my MacBook desktop). Plug your iPhone in to your laptop and launch iTunes. Select your device and go to Photos. Click the check box to Sync photos from and in the dropdown select Choose Folder and select your empty temp folder. Apply and sync. This instantly erased every picture I had on the iPhone.
Since I am not a fan of the Moments or Albums view Apple uses for iCloud/iPhoto sync, I created a local folder named Photos and then created individual folders inside of there (do not go more than 1 deep) and sorted my picture manually. The sub-folders are things like, "Cars", "People", "Beer" or whatever - and when you sync with your iPhone it keeps these folders to sort how you want them sorted - not how Apple likes them (by geotag or date).
If this trick works for you, please pass it on to someone that has the same problem. If it doesn't work for you, sorry...try Google...but please don't let me know as I am not affiliated with Apple in any way and will probably never check this thread again in my lifetime.