CSS sprites and pop up windows
I wouldn't approach it this way. Hardcoding list items, with content, in a tpl.php is generally frowned upon and will ultimately require more work than this should entail.

Instead use Views and the Colorbox module. Create a view with the list of nodes you want to display, pop it in a block on the page in the region you'd like it to display, and use the Colorbox plugin to Views show the popover on click. Use CSS to style the colorbox however you please. This can be handled strictly with configuration and a touch of CSS. This way, when the theme changes someday, the functionality is maintained.

See http://drupal.org/project/views and http://drupal.org/project/colorbox for more details.