Separate wallpapers out of repo about omakub

[dhh]

Actually, I think we're fine just leaving these 7 wallpapers in the back for now. If we get more sophisticated later with multiple choices or rotation we can revisit.

[orthotypos]

This is good idea generally, but also fwiw compressing these images on tinify give significant savings:

probably worth doing it regardless of the move to the new repo.

[dhh]

Those savings look suspiciously large. As-in quality taking a major hit to get there. But yes, otherwise I'm totally game to just reduce the size of the backgrounds.