So I\'m building a nested form with an Campaigns model and a Sites model where Campaigns has_many Sites. In my Campaigns form I have:
<%= f.fields_for :sites
You may want to investigate the paperclip gem: https://github.com/thoughtbot/paperclip
It lets you do this to show a thumbnail of the existing picture.
<% if @thing.logo? %> <%= image_tag @thing.logo(:thumb) %> Change <% end %> <%= f.label :logo %> <%= f.file_field :logo %>