weird caching problem

I'm calling an image through a php file, but when I make changes to the image, I can't get them to show. For some reason the old image is getting cached somewhere, and none of the standard remedies are working (refreshing, deleting cache, etc). What does work is changing the value of $icon and then reflecting that change in my code. But that's not ideal for what I'm doing. Any ideas? Here's how I'm calling the image in php: <?php //image.php { // some mysql stuff } if ($icon == 'a') header("Location: http://www.rcnchicago.com/~tortus/goombaw.gif"); else header("Location: http://www.rcnchicago.com/~tortus/goombab.gif"); ?> Thanks, Matt