Right now, it's pretty much just a static collection of cool pictures that Google Chrome Team pre-selected for us. Yes, we can also choose one that we previously downloaded but that's all.
So where are we going?
Answer is simple and obvious: Online.
Future versions of Google Chrome OS will come with a brand new Wallpaper Picker which will let us select online wallpapers. You will also be able to select online "media galleries" directly into your browser in order to change your wallpaper daily.
A "media gallery" is just a collection of media hosted somewhere in your Google Drive for instance.
My favorite collection of wallpapers is simpledesktops.com. Here's below the one I use for the moment.