Comment 3 for bug 1459988

It's a good question. There's a few more changes than that in the commited fix. Thanks to Rafael Laguna it was fixed. To get an exact overview of the changes made to fix it you can visit http://bazaar.launchpad.net/~lubuntu-art/lubuntu-artwork/quantal/revision/499

As for your second question I'm not sure how it works. But my guess is that the main brainch has not been uploaded to the ubuntu (codename vivid) repository.

$ apt-cache policy lubuntu-artwork-15-04

on Ubuntu vivid gives me

lubuntu-artwork-15-04:
  Installed: (none)
  Candidate: 0.54
  Version table:
     0.54 0
        500 http://se.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages

However, the latest version, 0.55, can be found in the ubuntu (codename wily) repository (see http://packages.ubuntu.com/search?keywords=lubuntu-artwork&searchon=names&suite=wily&section=all).