IE6 reports a nonsecure item error when hovering over menu items

Our Drupal site runs over SSL and uses SimpleMenu. When you mouse over the links that would normally have graphics, we're getting the error "This page contains both secure and nonsecure items. Do you want to display the nonsecure items?" Clicking More Info reveals nothing of value, just a generic help message about showing items from a non-encrypted source.

The images do not show up in any case. I have not been able to pin down the exact URL causing the issue, but I'll submit a patch if I can find the problem.