鼠标移入和移出button按钮时改变背景颜色，为什么移出不改变颜色呢？

<input type="button" value="上传" id="btnUpload" class="devbutton" onmouseover="mouseover()"
onmouseout="mouseout()" onclick="viewbillView(this,'<%=YPBH %>')" style="width: 80px;
height: 24px; border: #7F7F7F 1px solid; background-color: #F2F2F2; padding: 2px;
cursor: hand;" />

function mouseover()
{
document.getElementById("btnUpload").style.backgroundImage = "url(../../Css/Aqua/Editors/edtButtonCheckedBack.gif)";
}
function mouseout()
{
document.getElementById("btnUpload").style.backgroundColor = "#F2F2F2";
}
移入起作用，为什么移出执行这个function但不改变颜色呢？

相关了解……