app/inputs/img_input.rb

gistfile1.rb

#apps/views/seuform.html.erb
f.input :image_id, :as => :img, :collection => ActiveAdmin::ApplicationHelper.all_images,  :label => "Imagens"

=begin 
O Collection geralmente seria um map com o id e value de cada item, mas no meu exemplo, o value é a url da imagem 
e coloquei um terceiro valor para o endereço da imagem original que será usado no href do thumb da imagem.
=end

#/app/helpers/active_admin/application_helper.rb
def self.all_images
    Image.all.map {|image| [image.id, image.file.url(:thumb), image.file.url(:original)]}
end

class ImgInput
  include Formtastic::Inputs::Base
  def to_html
    object_name = self.object_name
    attribute_name = self.method.to_s
    id = "#{object_name}[#{attribute_name}]"
    html = "<li class=\"radio input optional\" id=\"#{object_name}_#{attribute_name}_input\"> <label class=\" "+
            "label\" for=\"#{id}\">#{self.options[:label]}</label> <div id=\"thumb_images\"  > "

    self.options[:collection].each do |image|
      href = "href=\"#{image[2]}\" target=\"_blank\"" unless image[2].nil?
      html << "<div style=\"margin-top:#{self.options[:margin_top]}px; margin-right:5px;heigth:80px; float:left;\">"+
          "<input  style=\"margin-left:5px;\" type=\"radio\" name=\"#{id}\" id=\"#{id}\" value=\"#{image[0]}\"/>"+
          "</div> <div style=\"float:left;\"> <label  for=\"#{id}\">"+
          "<a #{href} ><img class=\"img_thumb_image\" src=\"#{image[1]}\" /></a></label></div>"
    end
    html << "</div></li><br style=\"clear: both;\">"
    html.html_safe
  end
end