You can use an image button on the button
<input type=image src=PATH_TO_YOUR_IMAGE alt="Submit Me">
or set submit button background to an image using css
input[type=submit] {
background:url(BACKGROUND_IMAGE_PATH_HERE);
border:0;
display:block;
height:Change_TO_backgroundimageheight;
width: Change_To_backgroundimageWidth;
}