Try using this:
$(document).on('change', '.nivo-main-image', function () {
var src = $(this).attr('src');
$("#social-buttons > #pinterest").attr('href', src);
});
Edit, see comments below:
I added the following to the jquery.nivo.slider.js
file at line 209:
var src = $('.nivo-main-image').attr('src');
$("#social-buttons > #pinterest").attr('href', src);
It works perfectly now. (until the plugin gets updated )