Add :use_default_url options to the ImagePreviewInput class,

if set to true and the image field is nil it will fall back to the
  default_url method of carrierwave.

lib/simple_form_fancy_uploads/image_preview_input.rb

@@ -2,8 +2,10 @@ module SimpleFormFancyUploads
   class ImagePreviewInput < SimpleForm::Inputs::FileInput
     def input
       version = input_html_options.delete(:preview_version)
+      use_default_url = options.delete(:use_default_url) || false
+
       out = ''
-      if object.send("#{attribute_name}?")
+      if object.send("#{attribute_name}?") || use_default_url
         out << template.image_tag(object.send(attribute_name).tap {|o| break o.send(version) if version}.send('url'))
       end
       (out << super).html_safe

spec/dummy/app/models/page.rb

@@ -1,4 +1,5 @@
 class Page < ActiveRecord::Base
+  mount_uploader :avatar, ImageUploader
   mount_uploader :image, ImageUploader
   mount_uploader :attachment, AttachmentUploader
 end

spec/dummy/app/uploaders/image_uploader.rb

@@ -18,4 +18,7 @@ def extension_white_list
     %w(jpg jpeg png)
   end
 
+  def default_url
+    "fancy-upload-default-url.png"
+  end
 end

spec/dummy/app/views/pages/_form.html.erb

@@ -4,6 +4,7 @@
   <%= f.input :remove_image, as: :boolean if f.object.image? %>
   <%= f.input :attachment, as: :attachment_preview %>
   <%= f.input :remove_attachment, as: :boolean if f.object.attachment? %>
+  <%= f.input :avatar, as: :image_preview, use_default_url: true %>
 
   <div class="actions">
     <%= f.button :submit %>

spec/views/pages_views_spec.rb

@@ -38,6 +38,10 @@ def stub_file(filename)
     it "contains image preview" do
       rendered.should have_selector 'img[alt=Rails]'
     end
+
+    it "should fallback to default_url if specified" do
+      rendered.should have_selector 'img[alt=Fancy-upload-default-url]'
+    end
   end # uploaded image
 
   context "uploaded attachment" do