Untested example of post form in simple form

simple_form_post.html.erb

<%= simple_form_for [topic, post] do |f| %>
  <%= f.input :title %>
  <%= f.input :body, rows: 8 %>
  <% if post.image? %>
    <div class="form-group">
      <p>Current image</p>
      <%= image_tag( current_user.image.thumb.url ) %>
    </div>
  <% end %>
  <%= f.input :image, as: :file %>
  <%= f.hidden_field :image_cache %>
  <%= f.button :submit %>
<% end %>