Use remote_avatar_for

Carrierwave.html.erb

<%= form_for @user, :html => {:multipart => true} do |f| %>
  <p>
    <label>My Avatar URL:</label>
    <%= image_tag(@user.avatar_url) if @user.avatar? %>
    <%= f.text_field :remote_avatar_url %>
  </p>
<% end %>