What exactly do you want to do? Create your own animation frames? Or just disable the animation?
You can find layout rules for the animations here and from that you can figure out that to display a custom image frame during the animation in both directions you could write something like this (in addition to the list-style-images for the static images):
If you want to define your own animation frames then it would probably be easiest to create customized copies from chrome://browser/skin/stop-to-reload.svg and .../reload-to-stop.svg