the path is document relative. The correct path depends on the location of the page on your server.
If the page this css is on is at the root of the site:
and the iepngfix.htc file is in the css/css_images folder off the root of the site:
then yes, that is the correct code.
If you load the page in FireFox, and view source, links to css, js and image files are clickable, you can test if the path is correct by clicking the link to the iepngfix.htc file in source view, if the path is not correct, you will get a 404 page not found error.