css background image

hello, i have a problem, i have this menu which has divs as items, when you mouseover a div i want to change several properties of it using onmouseover=”this.className= bla bla bla…
everything works fine BUT when i try to change the background image, there is a delay displaying them, is there a way to save the images so that the browser gets them from the cache or something so they load faster instead of downloading it all over again?
this a fraction of the css code on my page

[CODE].nrm{font-size:12;background-image:url(‘src/nrm.gif’);width:100px;height:25px;background-color:navy;}
.ovr{font-size:12;background-image:url(‘src/ovr.gif’);width:100px;height:25px;background-color:#0066ff;}
.out{font-size:12;background-image:url(‘src/out.gif’);width:100px;height:25px;background-color:navy;}[/CODE]

hope this is clear enough
thanks^{^^}

Also in #CSS _↴