Don't know why the first one is not working.
The second one is not working because your script is not inside a dom ready handler, so when your event registration code is executed the target elements are not loaded in the dom, this results in the handlers not getting added to the target elements.
So add the script in a dom ready handler
jQuery(function ($) {
$('#lp-pom-image-xxx').click(function (event) {
event.preventDefault();
var buttonLink = $(this).attr("href");
var callbackFunction = function () {
window.location = buttonLink;
}
if (window._gat) {
_gaq.push(['_set', 'hitCallback', callbackFunction]);
_gaq.push(['_trackEvent', 'x', 'y', 'z']);
} else {
callbackFunction();
}
});
})