add paperclip gem

https://github.com/thoughtbot/paperclip
$ bundle install
add "avatar" to "user"

* add "avatar" to "user"
*
* $ rails generate paperclip user avatar
* User model
*
* attr_accessible :avatar
* has_attached_file :avatar

* $ rake db:migrate
update views

* form.file_field :avatar
* image_tag @user.avatar.url