To optimize loading time, I suggest to Use "PngOptimizer", which is available for windows.
It is able to shrink pngs without loosing any quality by up to 87%. Most images, especially the big ones get 20 to 40% smaller.
This is possible, because color for invisible images is stored, even if they are useless.
The entire NJoy-Skin gets smaller by 50KB, which is around 16%.