Also,have you made >
Lists, Menus, & Tabs > Menu > PopUpItem > Normal and disabled images with color ? if so,the color will color the popupgutter leaving a white block where your white popupgutter image shows through,that is not a part of the separator.These images should be left transparent unless your popupgutter is going to be the same color as them.
The popup gutter can only have a vertical separator due to the way windows renders the image.
This picture is the result of red images at >
Lists, Menus, & Tabs > Menu > PopUpBackground
Lists, Menus, & Tabs > Menu > PopUpItem > Normal and disabled