why can't I delete photos from my iphone 12

I wish to delete the photos on my iphone? what is the quickest and easiest way to do this?

I have a PC and have taken copies of the photos. I now wish to clear and just leave few. I have selected and deleted lots, but it would seem that they don't go. I have even emptied the Deleted Photos Album. I have turned the phone off and back on but it still will not let me delete. This is sooo annoying and time consuming. There has to be an easier way.

Looking forward to your solutions.

Judy