something like this - i just used some of the iamges from teamroom, change as you like
<xp:image url="/1_Day_deselected_24.gif" id="image1">
<xp:eventHandler event="onmouseover" submit="false">
<xp:this.script>
<xp:executeClientScript>
<xp:this.script><![CDATA[
var image = dojo.byId("#{id:image1}");
dojo.attr(image, "src", "1_Day_selected_24.gif");
]]> </xp:this.script>
</xp:executeClientScript>
</xp:this.script>
</xp:eventHandler>
</xp:image>