In a Ruby on Rails project by default the root of the HTML source for the server is the public directory. So your link would be:
<img src="images/rss.jpg" alt="rss feed" />
But it is best practice in a Rails project to use the built in helper:
<%= image_tag("rss.jpg", :alt => "rss feed") %>
That will create the correct image link plus if you ever add assert servers, etc it will work with those.