This article about CSS image replacement for submit buttons could help.
"Using this method you'll get a clickable image when style sheets are active, and a standard button when style sheets are off. The trick is to apply the image replace methods to a button tag and use it as the submit button, instead of using input.
And since button borders are erased, it's also recommendable change the button cursor to
the hand shaped one used for links, since this provides a visual tip to the users."
The CSS code:
#replacement-1 {
width: 100px;
height: 55px;
margin: 0;
padding: 0;
border: 0;
background: transparent url(image.gif) no-repeat center top;
text-indent: -1000em;
cursor: pointer; /* hand-shaped cursor */
cursor: hand; /* for IE 5.x */
}
#replacement-2 {
width: 100px;
height: 55px;
padding: 55px 0 0;
margin: 0;
border: 0;
background: transparent url(image.gif) no-repeat center top;
overflow: hidden;
cursor: pointer; /* hand-shaped cursor */
cursor: hand; /* for IE 5.x */
}
form>#replacement-2 { /* For non-IE browsers*/
height: 0px;
}