Debian (and Ubuntu) ship accountsservice with a patch that adds user backgrounds. It's useful for example on display managers like LightDM to allow them to show a different background for each user. Would it be possible to include their patch?
There was a bug 43578 report discussing this with a status "RESOLVED FIXED", but I don't see how it was resolved.
Link for Debian's patch:
The file is /patches/0011-add-background-file-support.patch.
I've updated LightDM to make use of the extension feature in AccountsService so it doesn't need this patch to be upstreamed.