Another option would be to add the onmouseout and on mouseover on page load. That is what I often do to keep from needing multiple CSS's or bloating one CSS since I may have several of these buttons on a site. This is under the assuption you utilize the 'typical' javascript for hover effects.
Example code:
protected void Page_Load(object sender, EventArgs e)
{
this.btnSubmit.Attributes.Add("onmouseout", "MM_swapImgRestore()");
this.btnSubmit.Attributes.Add("onmouseover", "MM_swapImage('" + this.btnSubmit.ClientID + "','','Images/Submit_h.jpg',1)");
}
The following Javascript must be placed in between <head></head> tags of the html or included in js file. They aren't all needed but I utilize preload and hover affects on almost everypage so I include all of these on most of my projects.
<script type="text/javascript">
<!--
function MM_swapImgRestore() { //v3.0
var i,x,a=document.MM_sr; for(i=0;a&&i<a.length&&(x=a)&&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.indexOf("#")!=0){ d.MM_p[j]=new Image; d.MM_p[j++].src=a;}}
}
function MM_findObj(n, d) { //v4.01
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[n];
for(i=0;!x&&d.layers&&i<d.layers.length;i++) x=MM_findObj(n,d.layers.document);
if(!x && d.getElementById) x=d.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))!=null){document.MM_sr[j++]=x; if(!x.oSrc) x.oSrc=x.src; x.src=a[i+2];}
}
-->
</script>