Here is the code I use for it:
{position:fixed; top:0px; left:0px;}
img {height: 0px; width: 100px; padding-bottom: 100px; background: url(URL HERE);}
this is the code i use. if the overlay is a different size than the default 64x64, then just change the 64 to whatever the dimensions of your new overlay are.
img {height:0px; width:64px; padding-bottom:64px; padding-top:0px; background:url(URL) left top no-repeat}