This demonstrates a commonly-asked question: how do I replace Backstretch's image once it's been called? Answer: easy, just call it again!
Furthermore, it gives an example of simple onclick functionality.
$.backstretch("chocolate-cups-landscape.jpg");
$("#cups").click(function() {
$.backstretch("chocolate-cups-landscape.jpg");
});
$("#tart").click(function() {
$.backstretch("chocolate-tart-landscape.jpg");
});