snippet.rb

class PhotoPicker # < Liquid::Tag                                            
    def initialize(tag_name, photo_pos, tokens)
       super 
       @photo = photo_pos.to_i
    end

    def render(context)
      #photos[@photo].image.url(:medium)
      context['post'][@photo].image.url(:medium)
      #rand(@photo)
    end
end 

  Liquid::Template.register_tag('picture', PhotoPicker) 
end

#index
<%= textilize(Liquid::Template.parse(post.content).render('post' => post.photos)) %>