I certainly don't want to list 4 million redirects in my nginx config files... what's the best practice with a large number of redirects like this? They'd be permanent as well as the old links live in a lot of customers' historical emails, so I can't count on Google et al updating their indexes over time.

I suppose I need to have a 404 set up that checks the JPG requested against a database of old URLs with their corresponding new URL?