regarding custom backgrounds in wordpress

regarding this article: https://premium.wpmudev.org/blog/daily-tip-how-to-enable-custom-backgrounds-for-wordpress-3-0-themes/ which is a really cool way to offer even more customization I would simply like to know where the uploads end up.
I mean how well is this working with a multi-site enabled installation? will users overwrite each others backgrounds if they upload files with the same names