background-image:url('/img/eg_bg_03.gif');
<script type="text/javascript">
if (event.srcElement.tagName=="INPUT")
event.srcElement.style.backgroundImage="url('/img/eg_bg_04.gif')"
if (event.srcElement.tagName=="INPUT")
event.srcElement.style.backgroundImage="url('/img/eg_bg_07.gif')"
<form onmouseout="out()" onmouseover="over()">
<input class="a" type="button" value="按钮 1"><br />
<input class="b" type="button" value="按钮 2"><br />
<input class="c" type="button" value="按钮 3"><br />