How Do You Change Button Image?

I have CSS for button like

input[type="button"] {
background-image: url(imageFile.gif);
background-repeat: no-repeat;
background-position: right center;
border-width: 0;
border-spacing: 0; padding: 0;
border-collapse: separate;
width: 15;
background-color: cyan;
}

and HTML

<form>
<input type="button" onClick="changeImage(this)">
</form>

and JavaScript

var newImage = new Image();
newImage.src = "newimageFile.gif";

function changeImage(thisbutton) {
thisbutton.style.xxxx = arrow_down.src;
}


My question is what will be for xxxx?

I try
..style.background-image for xxxx, but Netscape/Firefox and IE dosen't
like that.

..style.background is for the background-color, not for background-image.