Setting a PHP $_SESSION['var'] using jQuery

You can't do it through jQuery alone; you'll need a combination of Ajax (which you can do with jQuery) and a PHP back-end. A very simple version might look like this:

HTML:

<img class="foo" src="img.jpg" />
<img class="foo" src="img2.jpg" />
<img class="foo" src="img3.jpg" />

Javascript:

$("img.foo").onclick(function()
{
    // Get the src of the image
    var src = $(this).attr("src");

    // Send Ajax request to backend.php, with src set as "img" in the POST data
    $.post("/backend.php", {"img": src});
});

PHP (backend.php):

<?php
    // do any authentication first, then add POST variable to session
    $_SESSION['imgsrc'] = $_POST['img'];
?>

Might want to try putting the PHP function on another PHP page, and use an AJAX call to set the variable.