How to compress a huge amount of PNG images?

PNG compression is a lossless compression. However, a caveat to this is that there will be times when compressing an image will not always yield the results you want (% compression ratio won't be that great). In contrast to lossful compression (like JPG compression), where you will almost always get a filesize reduction (% compression ratio might be higher), but you actually lose image quality.

Have you tried PNG Crush?

PNG compression works better for images that have large areas of flat color, like cartoons that are colored with simple color fill, as apposed to a painting which has many colors which look similar, but really aren't.