You might not know this but when you delete a photo from your Camera Roll, it gets stored in the ‘Recently Deleted’ section temporarily for 30 days rather than getting deleted permanently. To delete it permanently, you have to remove the photo from that Recently Deleted folder as well.
You might not know this but when you delete a photo from your
Camera Roll, it gets stored in the ‘Recently Deleted’ section temporarily for
30 days rather than getting deleted permanently. To delete it
permanently, you have to remove the photo from that Recently Deleted folder as
well. 
DeleteForever jailbreak tweak lets you add an extra option to “Delete
Permanently” in the stock Photos app that makes very easier for you.
 After installing this tweak, there’s no option to configure.

With the tweak installed, whenever you want to remove the photos from
Camera Roll by tapping on the trash icon, an action sheet appears that allows
you to choose whether you’d like to delete the photo normally, where it will
get stored in the Recent Deleted section, or delete it permanently.
So this tweak enables you an option to delete it permanently rather than
moving it to the Recent Deleted section.
DeleteForever tweak is available in Cydia via the BigBoss Repo for free and
supports iOS 8 or higher. If your device has not yet jailbroken, then
here’s How to
Jailbreak your iPhone, iPad & iPad Touch
So, if you’re willing to try this tweak, Please share your opinions
about the tweak in the comments section below.