Adding a submit button image to a Rails form

I'm creating a form in Rails for submitting comments, and I want the submit button to be an image, rather than an HTML button. In this bit of documentation, it says the code is image_submit_tag, but I can't get it to work in my code.

Here's what I'm working with:

<% form_for [@post, Comment.new], :remote => true do |f| %>
<p>
    <%= f.label :body, "Add a comment" %><br />
    Name <%= f.text_field :name %><br />
    Website<%= f.text_field :website %><br />
    Twitter<%= f.text_field :twitter %><br />
    <%= f.text_area :body %>
</p>
<div id="comment-form">

    <div id="comment-button"><p>
        <%= f.image_submit_tag("comment-button.png") %></p>
        </div>
</div>
<% end %>

Thanks for the help.