As in - some of the images I've uploaded - like the ones at http://astroclimatology.wordpress.com/2011/11/02/interesting-dif-between-surface-temperature-and-p850-temperature/ - use images from a webpage that won't be here a year from now - as I'm leaving the school in a year.
The /wp-content/uploads directory contains them. But read on.
Backup consists of two elements:
The #2 is because some plugins and themes store settings in files, even if totally inadvisable (that never stopped them). And it's best to save everything just to stay on the safe side.
you get your all external images when you upload your database back in Your_Database->wp_posts->post_content you can see your content in html format so its clear that their are anchor tags also so you'll get your external images for sure.