I just had this issue myself. I think it's old photos in Photo Stream that are somehow left on the device. I figured this must be it because after deleting all my photos in Camera Roll, the count still showed the exact same amount as what I had in Photo Stream. This is the solution I found to clear the pictures off the device and get your space back (mine is an iPhone but the process would be the same for an iPod Touch or iPad). PLEASE NOTE: THIS PROCESS WILL DELETE ALL YOUR PHOTOS FROM YOUR PHOTO STREAM. So if you want to keep them, make sure to download them to a separate file on your computer before proceeding.
1) Go to Settings>Photos & Camera> Turn on iCloud Photo Library (if it's already on, go to next step).
2) Plug your device into your computer and go to "iCloud.com" and sign in with your iCloud ID and password. When the page loads, click on Photos (it will look the same as the app on your device). Wait for the photos to load. Once loaded, delete them all. (You can highlight them and delete them all in one shot.) When they are deleted, unplug your device and go to the next step. (If there are no photos and none have downloaded, go on to next step.)
3) Go to Settings>iCloud>Photos. Turn off My Photo Stream then turn back on (or turn it on if it is already off) and let all the photos in Photo Stream load (it may take a while, and even if you hadn't been using My Photo Stream, it may pick up some old ones and put them back). When they're loaded, delete them all. (You can turn off Photo Stream at this point if you don't want to use it).
Now if you go to Settings>General>About, the photo count should show only what you have in your Cameral Roll (zero if you have no pictures on the device at all).
I also went this extra step below to disable iCloud Photo Library. I don't think this should have an effect on showing the right count (I forgot to check the count before I went ahead with this step, so I'm not sure) but if the count is still wrong, try this step as well:
Go to Settings>iCloud>Storage>Manage Storage>iCloud Photo Library>Disable and Delete
Hope this works for you too.