gistfile1.js

<a id="last" href="#">last</a>
<img id="mug" count="4" src="mug4.png" />
<a id="next" href="#">next</a>

...

$('#last').click(function () {
  var mug = $('#mug');
  var count = mug.attr('count');
  count = (count - 1) % 48;
  mug.attr('count', count).src('mug' + count + '.png');
});

$('#next').click(function () {
  var mug = $('#mug');
  var count = mug.attr('count');
  count = (count + 1) % 48;
  mug.attr('count', count).src('mug' + count + '.png');
});