Deleting it off the phone doesn’t “delete it” from the phone. It just deletes the pointer that tells where it resides on the phone’s storage and frees that space for other uses. If you truly want to DELETE something from your phone, you need to delete it using the phone’s delete command, and then you need to fill up all available space on the phone with other things. Snap 5,000 pictures of the sky or anything else until the phone tells you that it’s out of storage space. That will overwrite the file you thought you deleted.
Or reformat the memory card.