Thanks for creating a plugin that fills a total void.
I looked everywhere for similar functionality and could not find it until I came across your plugin as a result of a WPMU.org post.
Installed it on one of our multisite installations with about 200 sites and another with about 50 sites and it worked perfectly except for needing site names to display in a drop down list. Currently only two or three sites are accessible.
I sent you a 125 Euro donation for your great plugin and request you add two features to it.
1) Convert list of available sites on the network to drop down list
2) Provide a means for some sites to be excluded or private so that admins who do not want to share their graphics can restrict access to their media library. On a large network, this might be a site-by-site admin selectable option. (i.e., YES NO Include my sites graphics in Network Media Library for others to use)
I look forward to these enhancements!