Recently I use your tool a lot, and it is already annoying that the ditherig actually instantly running when you select a method.
Simply just don't run, start, calculate (or how I can say it) the dithering; just select it. Add a button to actually activate the calculation, so there will be no issue about freezing. I think the best solution would be that adding two buttons; one to actually run the calculation and obviously a cancel button. This eliminate the issues with large images.