Imageapi Optimize - enabled libraries exec
so what does the second one do?


It'll optimise the already-optimised image from the previous plugin as best as it can. Which won't be much.


  I suppose it should be radio buttons rather then checkboxes.


Perhaps; but that would remove the ability to have combinations of different kinds of plugins (not just file size optimisers) in the future, so it might be a bit short-sighted to say that.