I have just being {{nsd}} tagging 100 unsourced images from the OsamaK list at http://commons.wikimedia.org/wiki/User:OsamaK/Tagging <spam>Go and help tagging!</spam>
For each image, i tagged <no source> with UserMessages.js trying to guess the user language, and for users with user page/seeming to be the authors/many images (they often have a bunch of unsourced images with the same license) i also notified them in their home wiki (these are old images, we can't expect they're checking their commons talk if they haven't contributed for months!).
The 'notifications' were a line from the localised image source template with a link to their commons talk page. My message may be nonsense, but i hope they will understand and figure out they should go to commons and (seeing the full template) source their images ;-)
Doing it i found out that: *My guesses were quite good (they *had* an account there). *It's fast to create a new account (i made 4 and used several more). *A good way to detect the user language/home wiki is to check-usage where is the tagged image being used. *Leaving messages on RTL is hard.