The problem is that your background images are applied to the element whose class is "left" regardless of what appears inside that element. As yet there is no reliable way to apply a style to an element based on something inside that element. The adjacent sibling selector
might do this, but I am not able to test it at work, and it doesn't work in Internet Explorer regardless. There are various solutions. I assume you want the blue arrow to be applied to what would normally be a link to the page you're already on. You can try creating another class called "this" or something, and alter your CSS thus (noting the use of shorthand):
Then you can get rid of the "background" styles on all of the "a" elements because they're unnecessary. You will then have to alter your HTML so that the class of any element you want to have the blue arrow is "this left" rather than just "this". If you're generating your HTML dynamically it should be fairly easy to have your favorite server side determine which elements this should be applied to.
If this doesn't work correctly let me know - I will be able to test a little more extensively after work!
Good luck!