gistfile1.txt

rails generate migration add_image_id_to_blog_posts image_id:integer
rake db:migrate

Override the blog category model and add:
belongs_to :image

Override the blog category form and add an image picker to it:
<div class='field'>
  <%= f.label :image -%>
  <%= render :partial => "/shared/admin/image_picker", :locals => {
        :f => f,
        :field => :image_id,
        :image => f.object.image,
        :toggle_image_display => false
      } %>
</div>