Delete Photos from iPhone
How to delete specific images without completely wiping the iPhone in forensically sound way?
What is the version of the iOS running ?! Normally on up-to-date iOS versions deleting, then purging from the trash would do it. The only trail remaining could be thumbnails or databases info if the files in question were shared.
Agreed with @passcodeunlock - have seen thumbnails of deleted photos, as well as similar artifacts from iCloud sync, even old deleted thumbs from other iCloud-connected devices.
I would say ensure the phone is not backing up any data to iCloud, photos in particular, make sure iCloud backups are disabled, and all applications/databases that have access to photos. Also ensure phone is running most recent iOS.
Image test phone, delete a few test photos, reimage the phone a few times with different methods and test to see if your deleted photos can be found.