How about each time we upload a folder with all the frames of the animation inside it. This way we could easily just pick up from the last frame and add more frames e.g.
[frame 1] - [frame 25] -Someone else has a turn--> [frame 26] - [frame 50] and so on.
The frames would be saved in one folder and then put together in the end.
(Were using .gif ,right?)