relative path preview fix for #728

This fixes the relative path but does not address the named route issue-- I'm not sure how to do that and pass the string interpolation path function the values from javascript...

`
"<img src='#{Rails.application.routes.url_helpers.display_content_path(0, :media_id => MMM, :width => WWW, :type => 'Graphic')}' />"
`
MMM should reflect the `media_id.val()` and WWW should reflect the `preview.width()`