Use visibility: hidden instead of display none. This will keep the elements width and height attributes but will make it not visible. Here is how I modified your code to make it work.
The CSS
#ddmenubg2{
visibility:hidden;
}
The javascript
$(function(){
$(window).scroll(function() {
if ($(this).scrollTop() > 75) {
$("#ddmenubg2:hidden").css('visibility','visible');
$("#ddmenubg2:hidden").fadeIn('slow');
}
else {
$("#ddmenubg2:visible").fadeOut("slow");
}
});
});