error with multiple image upload

You're trying to call a nested method on your parent model.

You have:

#app/models/product.rb
class Product < ActiveRecord::Base
   has_many :pictures
end

#app/models/picture.rb
class Picture < ActiveRecord::Base
   belongs_to :product
   has_attached_file :image
end

This means that you will have to call:

<% @products.each do |product| %>
   <% product.pictures.each do |picture| %>
      <%= picture.image.url %>
   <% end %>
<% end %>

--

You're receiving a noMethod error because you're trying to call image on your Product object (which doesn't exist):

<%= picture.image.url #-> doesn't exist %>

Fix

The fix is a little more involved than you realize.

Specifically, you have to ensure that you're able to add/create the nested Picture objects, which comes from the accepts_nested_attributes_for method:

#app/models/product.rb
class Product < ActiveRecord::Base
   has_many :pictures
   accepts_nested_attributes_for :pictures 
end

This will allow you to create the appropriate form & controller actions:

#app/controllers/products_controller.rb
class ProductsController < ApplicationController
   def new
      @product = Product.new
      @product.pictures.build
   end

   def create
      @product = Product.new product_params
      @product.save
   end

   private

   def product_params
      params.require(:product).permit(..., pictures_attributes: [:image])
   end
end

You'll then use fields_for:

#app/views/products/new.html.erb
<%= form_for @product do |f| %>
   <%= f.fields_for :pictures do |p| %>
     <%= p.file_field :image %>
   <% end %>
   <%= f.submit %>
<% end %>

This will allow you to save new Picture's / images with a new product, then using the code above, you'll be able to show them!!

Bonus

A pro tip I keep sharing is paperclip_defaults:

#config/application.rb
...
config.paperclip_defaults = {
   :styles => {
       :thumb    => ['100x100#',  :jpg, :quality => 70],
       :preview  => ['480x480#',  :jpg, :quality => 70],
       :large    => ['600>',      :jpg, :quality => 70],
       :retina   => ['1200>',     :jpg, :quality => 30] #-> is this correct formatting?
   },
   :path => ":rails_root/public/images/:id/:filename",
   :url  => "/images/:id/:filename"
}

Instead of including all the styles in your Picture model, you can set them as "defaults" in your config/application.rb - which will allow you to just call has_attached_file :image, changing the styles as you require:

#app/models/picture.rb
class Picture < ActiveRecord::Base
   has_attached_file :image #-> will use the defaults which can be overridden as required
end