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/* ADD THIS TO CSS */
div#preloaded-images {
   position: absolute;
   overflow: hidden;
   left: -9999px; 
   top: -9999px;
   height: 1px;
   width: 1px;
}

<!-- ADD THIS TO XHTML -->
<div id="preloaded-images">
   <img src="http://perishablepress.com/image-01.png" width="1" height="1" alt="Image 01" />
   <img src="http://perishablepress.com/image-02.png" width="1" height="1" alt="Image 02" />
   <img src="http://perishablepress.com/image-03.png" width="1" height="1" alt="Image 03" />
</div>