How to fix iPhone 5S reporting excessive space used for Photo Library?
apparently the "recently deleted"-feature in iOS8 is broken.
Photos you've deleted don't disappear after 30 days, they're just hidden on your device.
The storage being used isn't "phantom usage", since the photos are still safely stored.
The easiest short term solution is to set back your date on the phone month by month (until the release of iOS8), and your deleted photos will reappear in the "recently deleted"-folder. There you can manually and permanently delete them.

As long you regularly delete all "recently deleted" images, your storage won't clog up.

If you want to get an overview of the photos on your iphone just download iexplorer. There you'll see all of them, even those you can't see on your iphone.

Here's a blog post about the issue: https://blog.tmoehle.de/ios-photo-problem/ 

Cheers,