Create linked image if user logged in, otherwise just show image

Hey everyone.

RoR beginner here, so be gentle please. I'm stuck at a probably silly
problem and would appreciate any help I can get.

I'm trying to create a linked image to an RSS feed if the user is
logged in; if the user is not logged in, it should just show the image
without the link.

I read through the framework docs and figured

<%= link_to_unless (session['user'].nil? , image_tag("/images/help/
16px-Feed-icon.png", :size => "16x16", :border => 0), "/index/feed")
%>

would do the trick, but that only displays the image no matter if I am
logged in or not.

Thanks in advance!