Pngyu、ImageAlpha其实底层使用的是pngquant
使用pngquant后可以使用imageOptim再次优化
pngquant - 普通版本
pngquant-openmp - 多核支持，速度更快
pngquant-openmp
This makes pngquant faster in wall-clock time on multicore machines when one
image at a time is processed.
However, it increases total CPU time used, and thus it‘s not most optimal
for server-side and parallelized batch jobs which run many pngquant
instances at a time.
原文：http://www.cnblogs.com/luweimy/p/4092517.html