school-platform/app/views/users/_picture_form.html.erb

<%= bootstrap_form_for(@user) do |f| %>
  <%= render 'shared/error_messages', object: f.object %>

  <%= image_tag @user.picture.url if @user.picture? %>
  <%= f.file_field :picture, accept: 'image/jpeg,image/gif,image/png' %>

  <%= f.submit "Save picture", class: "btn btn-primary" %>
<% end %>

<script type="text/javascript">
  $('#user_picture').bind('change', function() {
    var size_in_megabytes = this.files[0].size/1024/1024;
    if (size_in_megabytes > 5) {
      alert('Maximum file size is 5MB. Please choose a smaller file.');
    }
  });
</script>