Okay, I definitely had a source on this image when I uploaded it. I have no idea where it went... Let me see if I can get it out of my uploads page via the API.

EDIT - Done. For the benefit of anyone else who runs into the source vanishing for whatever reason, find the upload in your My Uploads page, right-click the thumbnail to get the old upload form URL, add ".json" to the end, and then get it out of the "source" field. If you forget the ".json", you'll wind up back at the image page.