It’s easy enough to backup the text content of a Wordpress.com blog using Manage > Export to download it in XML format - this can easily be imported into any recent version of Wordpress. But if you want to backup all the images you’ve uploaded to Wordpress.com without downloading them one by one, download and install the excellent Firefox extension DownThemAll, then go to your Wordpress blog’s Manage > Media Library page and set DownThemAll to download all your image links on the page.