iOS7 Moving Backgrounds - file type and creating your own
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.