One simple way is to add the hover-images like this in the code.
Code:
<img src="hover.gif" width="0" height="0" alt="" />
What webmaster is saying is add this code somewhere else in your page replacing "hover.gif" with your hover images. Setting the width & height to 0 means the image won't be displayed, but the browser will still load the image from the webserver before it is needed by the hover. This means a copy of the image will already be on the users machine before it is needed which will remove the hover delay.
-Mike