dropdown menu conflicts with logo in IE!!

Im using the photograpic theme with horizontal dropdownmenu on the top.
When im using the dropmenu with subitems, the logo on the header disappears (only in IE). look it here: http://www.studioanra.nl/home/

problem: when you roll over a parent item in head menu (portfolio) and after that you roll over the logo, you will see dat the logo disappears. Who can help me with the frustrating problem?