Implement jQuery's deferred object for callbacks.
I've decided to not implement this.
While returning a promise would work for the main use case (when all descendant images have loaded), it would lead to an inconsistent API for handling the per-image callback (which would remain as a callback).
It could be done through progress handlers, right?
Hadn't seen that before, thanks.
However, I'm going to leave this closed before as it'd be inconsistent with jQuery's collection based APIs.
Added in 0249d71