Html.ActionLink as a button or an image, not a link

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 */
}

I like to use Url.Action() and Url.Content() like this:

<a href='@Url.Action("MyAction", "MyController")'>
    <img src='@Url.Content("~/Content/Images/MyLinkImage.png")' />
</a>

Strictly speaking, the Url.Content is only needed for pathing is not really part of the answer to your question.

Thanks to @BrianLegg for pointing out that this should use the new Razor view syntax. Example has been updated accordingly.