What you could do is to place a Background 'Sprite' Graphic into the Search Field and move it programmatically.

<div id="skinsitesearch" class="search" style="background-position: 0px 0px;"><span class="search" style="background-position: 0px 0px;"><dnn:SEARCH runat="server" id="dnnSEARCH" cssclass="SEARCH" Submit="<img src=&quot;images/search.gif&quot; border=&quot;0&quot; align=&quot;absmiddle&quot;>" showWeb="false" ShowSite="false" /></span>

<script type="text/javascript">

<!--
var SearchField = document.getElementById('dnn_dnnSEARCH_txtSearch');
var skinsitesearch = document.getElementById('skinsitesearch');
function ChangeBG1(){
 skinsitesearch.style.backgroundPosition = '0px 0px';
}
function ChangeBG2(){
 skinsitesearch.style.backgroundPosition = '0px -20px';
}
if (SearchField.addEventListener){
 SearchField.addEventListener('focus', ChangeBG2, false);
 SearchField.addEventListener('blur', ChangeBG1, false);
} else if (SearchField.attachEvent){
 SearchField.attachEvent('onfocus', ChangeBG2);
 SearchField.attachEvent('onblur', ChangeBG1);
}

-->
</script>