shouldn't be a problem.
below is the code I've used in the input tag of Act_CartButtonImage.html
<INPUT SRC="../images/but_buy1.gif" NAME="NETQUOTEVAR:BUTTONNAME"
BORDER="0" type="image" value="send" onMouseOver="MM_swapImage('NETQUOTEVAR:BUTTONNAME','','../images/but_buy2.gif',1)" onMouseOut="MM_swapImage('NETQUOTEVAR:BUTTONNAME','','../images/but_buy1.gif',1)" alt="submit" >
I found it easier to hard code where to find the images rather than use NETQUOTEVARS.
Also remember if you haven't used rollovers on your pages before that you might have to hard code the javascript below inbetween your <head> tags (also good to preload 2nd image in the body tag). Don't quote me on that, CDicken will be better placed to answer this.
When you come to upload you get a couple of warning messages but just click O.K as it doesn't effect the site in anyway.
All the best
Bangers
javascript to place between head tags>>
<script language="JavaScript">
<!--
function MM_swapImgRestore() { //v3.0
var i,x,a=document.MM_sr; for(i=0;a&&i<a.length&&(x=a[i])&&x.oSrc;i++) x.src=x.oSrc;
}
function MM_preloadImages() { //v3.0
var d=document; if(d.images){ if(!d.MM_p) d.MM_p=new Array();
var i,j=d.MM_p.length,a=MM_preloadImages.arguments; for(i=0; i<a.length; i++)
if (a[i].indexOf("#")!=0){ d.MM_p[j]=new Image; d.MM_p[j++].src=a[i];}}
}
function MM_findObj(n, d) { //v4.0
var p,i,x; if(!d) d=document; if((p=n.indexOf("?"))>0&&parent.frames.length) {
d=parent.frames[n.substring(p+1)].document; n=n.substring(0,p);}
if(!(x=d[n])&&d.all) x=d.all[n]; for (i=0;!x&&i<d.forms.length;i++) x=d.forms[i][n];
for(i=0;!x&&d.layers&&i<d.layers.length;i++) x=MM_findObj(n,d.layers[i].document);
if(!x && document.getElementById) x=document.getElementById(n); return x;
}
function MM_swapImage() { //v3.0
var i,j=0,x,a=MM_swapImage.arguments; document.MM_sr=new Array; for(i=0;i<(a.length-2);i+=3)
if ((x=MM_findObj(a[i]))!=null){document.MM_sr[j++]=x; if(!x.oSrc) x.oSrc=x.src; x.src=a[i+2];}
}
//-->
</script>