What about an indicator that is only visible to you? I like the idea brought up already, about showing how much you've cost the site, and how much you've paid for it. It might be interesting to show some stats like BitTorrent seeding next to your name, probably your "balance" and your ratio. But maybe only turn those on after the first time you donate, and definitely only visible to you.

Oh, and the alt text could say "Thank you for donating!", and possibly provide extra stats.

This is a really good point! I've got some network hardware, and other computer gear, which I'm too busy to actively use, and I'd like to share, but still kinda want to reserve for myself. I've thought, periodically, about how a community rental system could work to facilitate that. Low monthly fee, rentals are free, charge fees for overdue stuff. I'm not sure if it would end up being more like a library or a hackerspace.