This is the image I wanted to upload by URL (to replace a lower-resolution version). It's from a public domain book, and they have full scans of it (and I guess other old books) at retrobibliothek.de, which was given as the image source. (Maybe some aren't PD; if so, then it would just take the uploader checking on a per-book basis, if the files are new rather than higher-resolution versions of previously uploaded files.)

In my case, I just downloaded it and uploaded it from my computer, but if someone wants to copy over the other scans they have (whether we don't have them at all or we have them at a lower resolution, as was the case with this one), it would be more convenient if they could do it without having to download and upload each file.