Decide a persistent filename for default wallpaper

[Sergey "Shnatsel" Davidoff]

There should be a filename for the default wallpaper persistent across releases. It may be (and will be) a symlink (controlled by alternatives system).

[Sergey "Shnatsel" Davidoff (shnatsel)]

This is needed for Ubiquity and Plymouth, as well as defaults

[Danielle Foré (danrabbit)]

"default-background.jpg" ?

[Sergey "Shnatsel" Davidoff (shnatsel)]

It should be unique and preferably should not have a fixed extension, some apps go crazy on extension mismatch but most linux apps can detect mimetypes without extensions. Also it should be unique and not clash with stuff shipped by different packages.
/usr/share/backgrounds/elementaryos-default will probably do

[Sergey "Shnatsel" Davidoff (shnatsel)]

"/usr/share/backgrounds/elementaryos-default" it is.

[Pim Vullers (pimvullers)]

Pantheon-greeter should use this link instead of the 16.jpg default background hardcode now.