<script type="text/javascript">
// 프리로드 이미지
var preloadImage = function(images) {
        if (document.images) {
                var i = 0;
                var imageArray = new Array();
                imageArray = images.split(',');
                var imageObj = new Image();
                for(i=0; i<=imageArray.length-1; i++) {
                        //document.write('<img src="' + imageArray[i] + '" />');// Write to page (uncomment to check images)
                        imageObj.src=imageArray[i];
                }
        }
};

preloadImage("/ms/images/sliders/main1.jpg,/ms/images/sliders/main2.jpg,/ms/images/sliders/main3.jpg"); // 미리 불러올 이미지 경로를 ,를 붙인 문자열로 작성하여 함수 호출
</script>

[출처]

http://blog.naver.com/enterpia/220252327952