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
From the form helper object (in your case builder and f) you should be able to access the model object and the url your looking for like this:
<%= image_tag f.object.image_url(:thumb) %>
The image in image_url depends on how you named the attribute, but for your sample this should be correct.