It's easy enough, to use an :
input[type=submit] {
background: transparent url("path/to/image.jpg") 0 0 no-repeat;
font-weight: bold;
display: inline-block;
text-align: center;
cursor: pointer;
height: 331px; /* height of the background image */
width: 500px; /* width of the background image */
border: 5px solid #fff;
border-radius: 4em;
}
JS Fiddle demo.