http://css-tricks.com/snippets/css/css-only-image-preloading/
Technique #1
Load the image on the element's regular state, only shift it away with background position. Then move the background position to display it on hover.
#grass { background: url(images/grass.png) no-repeat -9999px -9999px; }
#grass:hover { background-position: bottom left; }
Technique #2
If the element in question already has a background-image applied and you need to change that image, the above won't work. Typically you would go for a sprite here (a combined background image) and just shift the background position. But if that isn't possible, try this. Apply the background image to another page element that is already in use, but doesn't have a background image.
#random-unsuspecting-element { background: url(images/grass.png) no-repeat -9999px -9999px; }
#grass:hover { background: url(images/grass.png) no-repeat; }