.submit {
height: Xpx;
width: Xpx;
background-color: transparent;
/*gets rid of the white background color that
is there by default*/
background-image: url(../images/image.png);
_background-image: url(../images/image.gif);
background-repeat: no-repeat;
border: none;
text-indent: -5000px;
text-transform: uppercase;
/*don't ask, but this gets rid of the
dash in IE7 and 6*/
_display: block;
/*displays the image in IE6*/
_font-size: 0px; 
/*gets rid of the value because apparently the
text-indent doesn't do a damn thing in IE6*/
}

http://jc-designs.net/blog/2009/08/background-image-on-submit-button-in-ie6/

Taken from (http://jc-designs.net/blog/2009/08/background-image-on-submit-button-in-ie6/)

CSS input submit background image

css, image, background

CSS