When you delete a photo, it goes into the Recently Deleted Album for 30 days before it is automatically and permanently deleted.
If you don't want to wait 30 days for the photo to be deleted automatically, you can go into the Recently Deleted album, select the photos that you want to permanently delete and delete them manually.
Tap Photos
Tap Albums
Scroll down and tap Recently Deleted
Tap on the photo(s) that you want to delete immediately
Tap Delete
Make sure that you really want to delete the photo(s), since they cannot be recovered once deleted