The best way is to set the background transparent.

 border: none;
    background: transparent;
    cursor: pointer;

or

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;
}