Hi Timothy

There isn't a specific option for it, yet. But with some small CSS this would be possible. If you navigate to Appearance > Customize > Theme Options > Custom CSS you can paste the following in to the textarea:

.404-image {
    background: url(http://domain.com/path/to/image.jpg);
    width: 240px;
    height: 75px;
    margin: 0 auto;
}
.text-404 img { display: none; }

If you change the background of the 404-image selector and the width and height it will display your custom graphic.