You're talking about wallpapers compatible with parallax scrolling, I believe... yeah, those are a lot tougher to find/make than you'd imagine.
There's barely any media out there for wallpapers that support the MacBook Pro Retina displays, which is what I would likely have to make them from to get parallax-friendly wallpapers. Some of the ones posted have a higher resolution than the screen and will work with parallax on, though not ideally.
And it seems most GAF users turn that off on their phones anyways, so it seemed like catering to a crowd that doesn't exist.
That being said, if people would like to post parallax-friendly wallpapers, they are welcome to do so, but as with all the others, I will require that they be in the proper dimensions for ideal visual output.