iOS7 Moving Backgrounds - file type and creating your own iOS7 comes with parallax backgrounds. 

My questions are:
Are these a standard file type? Does anybody know what it is?

Is there a process to create a custom one?

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EDIT

Thanks everyone - I was asking for both. 

Edit - Someone has suggested that the dynamic backgrounds that are animated according to their programming  are quartz composer files. Is it possible to write one of these yourself and upload it to the phone? What would the process for this be?
You are confusing parallax and dynamic.

Parallax is the name given to the effect whereby a still image is moved in response to input at a greater or lesser extent than other on screen elements, to provide a sense of depth. The image used is just a static image file that is slightly larger than the screen size, to allow it to move slightly without introducing blank areas.

Dynamic backgrounds are the animated ones that move according to their programming, not just your input. I suspect they are simply quartz composer files like an OSX screensaver.