Another alternative is to use the <a href="{url-photon}">Photon</a> functionality that comes with the <a href="{url-jetpack}">Jetpack</a> plugin. It generates thumbnails on-demand using WordPress.com's infrastructure. <em>Disclaimer: The author of this plugin, Regenerate Thumbnails, is an employee of the company behind WordPress.com and Jetpack but I would recommend it even if I wasn't.</em>