paperclip is an awesome plugin for rails. it let's you attach an image to any of your models. installation and usage more information is available at http://jimneath.org/2008/04/17/paperclip-attaching-files-in-rails/

by default paperclip uses the image magick library for image manipulation/resizing. you'll need to install this library as well as the ruby api to it in order to use it. on ubuntu using apt get here are the commands

apt-get install imagemagick
apt-get install librmagick-ruby1.8