How it works
A photo can outlive a deletion. Deleting your own post removes your copy, but it cannot reach the others.
A friend can save a copy, someone can share it on another page, a website can keep an old page, and a search can keep a small preview.
Deleting still helps, because fewer copies is better. Words last as long as pictures too.
So use the future me test, and picture yourself at fifteen and at twenty-five.