urgent help please. Need to change the img:hover in the t3-sidebar-1

Hi guys,

i have build a menu with images (no text links) in the sidebar-1 . If i go over these images i get the standard hover color as background.

I try to ad these code in the custom.css :

.t3-sidebar-1 a:hover {

        background-color: #ffffff;

        }

but this dont helps.

i need a white background for every image hover.

what can i do?

thanks

Michael