Html.ActionLink as Image

Late response but you could just keep it simple and apply a CSS class to the htmlAttributes object.

<%= Html.ActionLink("Button Name", "Index", null, new { @class="classname" }) %>

and then create a class in your stylesheet

a.classname
{
    background: url(../Images/image.gif) no-repeat top left;
     display: block;
     width: 150px;
     height: 150px;
     text-indent: -9999px; /* hides the link text */
}