[yaix]

Yes, caching is only for the exact subset.

But you could do your own little client side cache. When receiving the url/img-data pairs, just store them in the HTML5 browser storage database, as key:url/value:img-data pairs. Then you are independent from the grouping into subsets.

And before requesting your next image sprite, just check with the client side data storage what images you already have and which you need to request.