Function image_name

pub fn image_name(kind: Kind, format: &str, index: usize, of: usize) -> String

What to call one image of a given kind, when several may share it.

back.jpg for the only back, booklet-01.jpg and booklet-02.jpg for the pages of an insert. The number starts at one and is only added when there is more than one of a kind, because booklet-01.jpg alone reads as a page torn out of something.