change menu item background

I need to change the background of a menu item (with li tag) if the user is in a specified category. I’ve allready got a little bit of code going to check if the user is in the specified category but now I need to change the background image.

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

 <?php if ( in_category('26') ) { ?>
           <!-- THE CODE FOR BACKGROUND IMAGE -->
 <?php } else { ?>
 <?php } ?>

 <?php endwhile; else: ?>

 <?php endif; ?>