Note this thread is out of date; we implemented preloading in canvas 2D via a hack, so even canvas 2D browsers like Safari should preload images layout-by-layout without loading anything during the game. So I am doubtful image loading is still causing any issues you're having.
Splitting up animations will have no effect whatsoever; if a sprite object is on a layout, the preloader will load every animation frame for it. As I said earlier in the thread I still strongly recommend you don't use such long animations, since it can be extremely wasteful for devices with limited RAM, like mobiles. We get a lot of support requests from people with "broken" games, when all they've done is given the device more images than it can fit in memory.