The Photos that Don't Go Away
Imagine the following scenario: you use your iPad or iPhone to take pictures with iOS 8, but you don't use the cloud for storing your photos. Eventually, those image files will start taking up the space on your phone. This happens frequently for me, so I connect my phone to my computer via a USB cable and use Windows Explorer to transfer the files to a directory on my hard drive for backup. Then, I only keep the photos that I want to have available on my device on the device.
This has worked very well until iOS 8, where we live in the dark days of Apple functionality. For reasons that may never be clear to me, Apple engineers replaced an intuitive, very well-functioning, and easy-to-use photo viewing interface with a train wreck in iOS8. The biggest problem comes when it is time to view photos- there is not yet a setting that allows the user to see all of the photos in one "bucket" or stream, just like there has been for every prior version of iOS.
If you find yourself in a situation where you are trying to delete picture content to free up space on your device, you may find the same surprise that I did; no matter how many pictures you delete, your usage data in Settings>General>Usage will never change. This is because deleted photos now go to a deleted photos folder, and you have to delete them twice before they are actually deleted.
Get to your photos by using the rainbow-colored Photos app icon, or by using the shortcut in the lower left of the camera app. Select "all photos" and look for a screen that gives you choices of "Photos" and "Albums" at the bottom. Select "Albums" and look for "Recently Deleted." Select that folder, and then select "Delete All" in the lower left corner. Now your usage data should reflect more storage space available.
This "recycle bin" arrangement may help if you accidentally delete a photo and need to recover it, but if you don't need that function this unwanted service may catch you by surprise.