Reading their terms of service, the only thing you can get via automated means is the XML RSS feed. Even then, it feels like they don't really want you to do be doing that to pull down high-res backgrounds as they discuss how it would affect the site.
This web site is intended to be accessed via standard web browser
software such as Apple Safari, Mozilla Firefox, Microsoft Internet
Explorer, and similar products via direct interaction by a human. With
the exception of publicly accessible RSS feeds provided in XML format,
the web site and its associated files are not meant to be accessed via
any automated means such as by scripts or bots or 3rd party
Be aware that if you utilize an automated means of accessing or
downloading this web site, in whole or in part, your access to the
site may be prevented, terminated, delayed, or slowed either
temporarily or permanently, especially if you attempt to download more
than 3 large files simulateously. This is necessary in order to
protect the user-experience of the web site for those who access it in
the manner envisioned by it's authors and architects.
Please understand that automated access to the site, via scripts,
bots, or other similar means can have the effect of seriously
degrading the performance of the web site or incurring significant
additional costs to its operators without sufficient revenue generated
to cover those costs. Keep in mind, that even minor infractions
against this policy can have a large negative effect when combined
with similar actions by other users.
We ask that you respect the above guidelines so that we may continue
to offer the web site as a free resource to the world. We prefer to
use our resources, both human and financial, to improve and expand the
features and content of the web site. Your cooperation is essential.
That said, if they would allow you to do that it would probably require a custom job to run and create the necessary file to point to. I don't believe there is anything already available to do this (I've briefly searched)