assets.js.erb

// helper to create proper asset paths if an asset host is configured
App.assets = {

  assets : {
    <% AssetsUtil.images.each do |img| %>
      "<%= img %>" : "<%= asset_path(img) %>",
    <% end %>
  },
  
  
  url : function(path) {
    return window.ASSETS_URL ? window.ASSETS_URL + path : path;
  },
  
  asset_path : function(source) {
    return this.url("/assets/"+source);
  },

  imageUrl : function(imageName) {
    if(this.assets && this.assets[imageName]) return this.assets[imageName];

    return this.asset_path(imageName);
  },
  
}

assets_util.rb

module AssetsUtil

  def self.assets_url
    self.config["environments"][Rails.env]["assets"]
  end
  
  def self.server_host
    self.config["environments"][Rails.env]["host"]
  end

  def self.config
    @@config ||= YAML.load_file(File.join(Rails.root, 'config', 'assets.yml'))

    @@config
  end

  def self.images
    Dir.glob(Rails.root.join("app/assets/images/**/*.*")).map do |path| 
      path.gsub(Rails.root.join("app/assets/images/").to_s, "")
    end

  end
  
end

example.ejs.jst

<div class="example">
  <img src="<%= App.assets.imageUrl('bookmarklet/add.png') %>">
<div>