The source url is supposed to point to an image's original address, and it's filled in automatically when uploading by url. That's the way we like it, but often we want to see the page an image was originally posted in, and it's not always easy to go from the url of the image to that of the page. Actually, it's never easy: it tends to range between tedious and impossible. How about adding a context url field, then? For examples, images that come from blogs, we could put the url of the blog entry in there (useful if you want to read the artist's comments); for post #380315, it would be an url to the auction; and so on. A Firefox extension for uploading to Danbooru could even fill in the url automatically, in many cases. The pixiv url filter thing could be changed to write to the new field, too.
Updated by aldeayeah